The object of this study was to determine the radiation effect on the urinary bladder and to establish the basic date for optimal fraction schedule on the whole abdominal irradiation of the mice. Although radiation damage of the urinary bladder is one of the dose-limiting factor for treatment of lower abdominal cancer, such as uterine cervical or rectal cancer, systematic histopathological study of total dose and recovery duration is very rare, especially in conventional fractionation regimen of clinical use. Authors used 198 mice and analyzed histopathological findings according to total dose (40&50 GY) and recovery duration (1-15 weeks after completion of irradiation) The results were summarized as follows: 1. No definite difference of radiosensitivity was noted between male and female group. 2. Most of mucosal injuries were recovered within 14 weeks in 40 GY irradiated group. 3. Vascular injury and change of connective tissue were prominent and persisted even mild degree until 15 weeks after completion of irradiation in 50 GY irradiated group. 4. Although follow up duration of this study (105 days) was not enough to compare life span of mice, this study emphasized that precious schedule for treatment planning was necessary for preventing or reducing of late complication.