Abstract: Objective To investigate the impact of rectum filling status on the dosimetric parameters of target volume and organ at risk （OAR） in intensity-modulated radiotherapy （IMRT） for patients with prostate cancer. Methods Fifteen patients with prostate cancer were enrolled in this study. Target and OAR were contoured by the same doctor both when the rectum was full and empty. And the IMRT plan was designed by the same medical physicist at the same radiotherapy dose. The impact of rectum filling status in dose of target and OAR during radiotherapy was investigated. Results If the rectum filling status was consistent in orientation and radiation， the rectum fill status was not associated with the dosimetric parameter of target volume and bladder， femoral heads （P＞0.05）. If the rectum filling status was not consistent in orientation and actual radiation， the dosimetric dose， uniform index and conformal index of target were different（P＜0.05）. The impacts on bladder and femoral heads were not significant. ConclusionTo guarantee the perfect execution of IMRT in patients with prostate cancer， it is recommended to keep the same rectal filling status during orientation and actual radiation.