Background: Psoriasis is a chronic inflammatory skin disease that is estimated to affect 0.6% to 4.8% of the general population. The optimum number of NB-UVB phototherapy sessions is yet to be determined. The aim of this study was to compare therapeutic effects of NB-UVB in treating chronic plaque type psoriasis three times weekly vs. five times weekly. Materials and Methods: This study was designed as a single-blinded randomized controlled trial. Patients with chronic plaque type psoriasis who were candidates for phototherapy were randomized in two groups, one group received NB-UVB three times a week (group A) while the other group received the same treatment five times a week (group B) . Patients’ PASI score was determined before commencing the study and after 12 weeks of treatment. At the end of the study, response rate, PASI score of patients and total dose of UVB per patient were determined. Results: A total of 47 patients were enrolled. Thirty two patients (19 in group A, 13 in group B) completed the study. Total response rate was 71.9% after 12 weeks of treatment (23 out of 32 patients) while nine patients (28.1%) had no response. Response rate was 78.9 % (15 out of 19) in group A Vs. 61.5% (8 out of 13 patients) in group B (p>0.05). Total UVB dose received, treatment sessions, erythema and final PASI score was lower in group A (p>0.05). Conclusion: With similar therapeutic effects, it seems that three times a week regimen may be superior to five times a week for its lower total UVB dose received and higher safety profile.