Objective: Mycosis fungoides (MF) is the most common primary cutaneous lymphoma. Mycosis fungoides often develops slowly over many years, presenting with a generalized erythroderma, skin patches or plaques. The diagnosis of MF requires the integration of clinical and histopathologic findings. Narrowband UVB (NBUVB) is widely used to treat MF. To evaluate the effectiveness of therapy the histopathologic findings before and after NBUVB reviewed using GUITART criteria. Method: We enrolled 20 patients (12 women, 8 men; age range, 10-80 years; mean age, 45.5 years) with clinically and histologically proven MF. The patients received NBUVB phototherapy three times a week. A biopsy was performed 3 months after onset of the treatment and GUITART criteria used to scoring MF before and after therapy. Results: Phototherapy was reduced the primary intraepidermal atypical lymphocytes (P value=0.008), dermal atypical lymphocytes (P value=0.008), epidermotropism (P value= 0.001), density of infiltration (P Value=0.002) and lymphocytic infiltrate without inflammatory features (P Value = 0.046) in all patient. But the relationship between the reticular fibroplasia of papillary dermis and phototherapy was not proved (P value=0.18). Overall score of patients was significantly lower after phototherapy (P value=0.000). Conclusion: Our data suggest that NBUVB therapy is reduces histiologic score of MF and is effective for the treatment of Iranian patients.