Background: Phototherapy is an important modality in dermatology and the number of skin diseases which can be controlled with it is increasing. In order to start treatment, the first dose of phototherapy is determined by measurement of minimal erythema dose (MED) in each patients individually or according to patient’s skin type. Objective: To determine the relationship of skin type with MED in patients under UV phototherapy referred to Razi hospital in Tehran. Patients and Methods: In patients referred to Razi hospital in 1381 (2002-03) for UVB phototherapy, skin type was determined according to Fitzpatrick’s classification and MED was measured using a UV-meter. Age, sex and the disease of the patients were also recorded. The data were analyzed using one way ANOVA and Welch tests. Results: Eighty patients including 39 males and 41 females were studied. The mean age of the patients was 32 years (SD=15.8). Their diseases include psoriasis, vitiligo, parapsoriasis, pityriasis lichenoides chronica, perforating disorder and patch stage mycosis fungoides. The mean of MED in 17 patients with skin type II was 111.8±39.2, in 42 patients with skin type III was 105.9±43.1, and in 21 patients with skin type IV was 114.3±39.2 mJ/cm², with no significant statistical difference. Conclusion: The first dose of phototherapy could not be determined based on skin type and measurement of MED is necessary in each patient.