Objective To observe the bladder injury in the three-dimensional conformal radiation therapy of spontaneous canine benign prostatic hyperplasia , and to provide a basis for clinical. Methods eight qualified BPH male canine models which were 7-15 year-old were chosen. The prostate directly exposed to 3-DCRT.The whole prostate of per animal was the target and sorrounded by 90% isodose curve,and was subjected to irradiation of 6MV X-ray by 2Gy per time and the total dose of 14 Gy. The bladder volume and dose of irradiation were measured with DVH. 1 month after irradiation, the exposured bladder next to the prostate and non-irradiated regions of the bladder (as control) were taken as biopsy. The bladder biopsy for optical microscopy, scanning and transmission electron microscopy. Result After irradiation,no significant change of urinating occurred,and drugs, treatment were not given. According to RTOG acute radiation injury standard, the damage were evaluated as grade 0 to 1. Compared with the pre-irradiation, we could see by optical microscopy： obvious necrosis, disorganization, suspension off of the epithelial cells and hole left behind the partial detachment of the epithelial cells,the disordered muscle and bare fiber.and we could see by transmission electron microscope: the connection of damaged, epithelial cells detached,proliferation of collagen fibrils and radiation fibroblasts. Conclusion The pathological change of radiocystitis such as necrosis and shedding of epithelial cells, collagen proliferation, and radiation fibroblasts occurred after irradiation in the dose lower than the currently accepted thresholds.