**Exhibit 12.1**

**CERTIFICATIONS OF MITTAL STEEL COMPANY N.V.’S CHIEF EXECUTIVE OFFICER AND**

**CHIEF FINANCIAL OFFICER PURSUANT TO RULE 13a-14(a) UNDERTHE EXCHANGE ACT**

I, Mr. Lakshmi N. Mittal, certify that: 

 

2.      Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances underwhich such statements were made, not misleading with respect to the period covered by this report; 

3.      Based on myknowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the Company as of, and for, the periods presentedin this report; 

4.      The Company’s other certifying officer(s) and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the Company and have: 

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensurethat material information relating to the Company, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared; 

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under oursupervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles; 

(c) Evaluated the effectiveness of the Company’s disclosure controls and procedures and presented in this report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and 

(d) Disclosed in this report any change in the Company’s internal control over financial reporting that occurred during the period covered by the annual report that has materially affected, or is reasonably likely to materially affect,the Company’s internal control over financial reporting; and 

5.      The Company’s other certifying officer and Ihave disclosed, based on our most recent evaluation of internal control over financial reporting, to the Company’s auditors and the audit committee of the Company’s board of directors (or persons performing the equivalent functions):

(a)    All significant deficiencies and material weaknesses in the design or operation of internal control over financial reportingwhich are reasonably likely to adversely affect the Company’s ability to record, process, summarize and report financial information; and 

(b)    Any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal control over financial reporting. 

Date: April 17, 2007 

 

Chairman of the Board of Directors and Chief Executive Officer of Mittal SteelCompany N.V. 

I, Mr. Aditya Mittal, certify that: 

1.      I have reviewed this annual report on Form 20-F of Mittal Steel Company N.V.; 

2.      Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances underwhich such statements were made, not misleading with respect to the period covered by this report; 

3.      Based on myknowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the Company as of, and for, the periods presentedin this report; 

4.      The Company’s other certifying officer(s) and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the Company and have: 

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensurethat material information relating to the Company, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared; 

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under oursupervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles; 

(c) Evaluated the effectiveness of the Company’s disclosure controls and procedures and presented in this report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and 

(d) Disclosed in this report any change in the Company’s internal control over financial reporting that occurred during the period covered by the annual report that has materially affected, or is reasonably likely to materially affect,the Company’s internal control over financial reporting; and 

5.      The Company’s other certifying officer and Ihave disclosed, based on our most recent evaluation of internal control over financial reporting, to the Company’s auditors and the audit committee of the Company’s board of directors (or persons performing the equivalent functions):

(a)    All significant deficiencies and material weaknesses in the design or operation of internal control over financial reportingwhich are reasonably likely to adversely affect the Company’s ability to record, process, summarize and report financial information; and 

(b)    Any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal control over financial reporting. 

Date: April 17, 2007 

 

/s/ ADITYA MITTAL
 
Mr. Aditya Mittal
 
Chief Financial Officer, Member of the Group Management Board, Flat ProductsAmericas.