Background & Aims: The main aim of this study was to evaluate the effect of the photon energies (6 and 15 MV) and the number of radiation fields (3 fields and 4 fields) on tumor dose distribution and dose received by organs at risk in radiation therapy of bladder cancer. Materials & Methods: In this study, CT images of 12 patients with bladder cancer were used. And 8 different radiation treatment plans were designed for each patient (with energies of 6 and 15 MV and 3 and 4 fields), and a total dose of 6100 cGy (4500 cGy for pelvic and 1600 cGy for bladder) was used. Results: The results showed that the use of 15-MV energies for radiation of the pelvic and bladder using 3 radiation fields lead to maximum uniformity and homogeneity of dose in the irradiated tumor tissue. The results also showed that the use of 15-MV energies for radiation of the pelvic and bladder and the use of 4 radiation fields in patients decrease the average integrated dose in the prostate and femur heads. However, based on the results obtained to reduce the average integrated dose of rectum in bladder radiation therapy, 3 radiation fields with energy of 15 MV for pelvic and 6 MV for bladder should be used. Conclusion: The results indicate a significant effect of photon energy and the number of radiation fields on distribution of dose in the tumor tissue and also the amount of dose received by organs at risk in radiotherapy of patients with bladder cancer. SOURCE: URMIA MED J 2015: 26(7): 560 ISSN: 1027-3727