Aims: The aim of this study was to evaluate the effects of gamma irradiation on the disinfection of the water and wastewater from Isfahan wastewater treatment plants. Materials and Methods : Using a 60 CO gamma radiation machine with emission rate of 405.38 CGy/min, water and wastewater samples were irradiated at doses of 20-160 Gy and 80-240 Gy, respectively. Microbial cultures were performed on the samples to assess the pre- and post-irradiation coliforms content. Results : The results of microbial tests showed that approximately 100% of the total and fecal coliforms in water samples were inactive at 160 Gy dose of gamma radiation. At a dose of 240 Gy, gamma-ray efficiency for inactivation of the total coliforms in the effluent was different, considering the sample quality. The efficiency for secondary effluent, polished, and filtered samples was 56, 83, and 64%, respectively. In this case, reduction of fecal coliforms was 46, 58, and 81%, respectively. The effective reduction of the coliforms in the effluent samples (80-50%) was also observed at doses of 120-240 Gy. According to the dose-response curves, as the radiation dose increases, the inactivation of the coliforms increases linearly. Conclusion: At gamma doses used in this study, the coliforms were removed completely. But for wastewater samples, although the removal efficiencies were above 80%, the environmental standards related to microbial parameters were not satisfied. Therefore, it is necessary to use higher doses of gamma radiation.