<PAGE>
                                                                    Exhibit 12.1


CERTIFICATIONS OF MITTAL STEEL COMPANY N.V.'S CHIEF EXECUTIVE OFFICER AND CHIEF
FINANCIAL OFFICER PURSUANT TO RULE 13A-14(A) UNDER THE EXCHANGE ACT

I, Mr. Lakshmi N. 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 under which such statements were made, not
misleading with respect to the period covered by this report;

3. Based on my knowledge, 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 presented in this report;

4. The Company's other certifying officer and I are responsible for establishing
and maintaining disclosure controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) 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 ensure that 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) Evaluated the effectiveness of the Company's disclosure controls and
      procedures and presented in this report our conclusions about the
      effectiveness of the disclosure controls and procedures, as of the end of
      the period covered by this report based on such evaluation; and

      (c) 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 I have 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 reporting which 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: March 31, 2005



/s/  Mr. Lakshmi N. Mittal
-------------------------------------------------------
Mr. Lakshmi N. Mittal
Chairman of the Board of Directors and Chief Executive Officer of Mittal Steel
Company N.V.
<PAGE>
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 under which such statements were made, not
misleading with respect to the period covered by this report;

3. Based on my knowledge, 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 presented in this report;

4. The Company's other certifying officer and I are responsible for establishing
and maintaining disclosure controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) 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 ensure that 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) Evaluated the effectiveness of the Company's disclosure controls and
      procedures and presented in this report our conclusions about the
      effectiveness of the disclosure controls and procedures, as of the end of
      the period covered by this report based on such evaluation; and

      (c) 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 I have 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 reporting which 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: March 31, 2005



/s/ Mr. Aditya Mittal
---------------------------------------------
Mr. Aditya Mittal
President and Chief Financial Officer of Mittal Steel Company N.V.