Preoperative radiation therapy has been regarded as the optional neoadjuvant treatment to decrease local recurrence of rectal cancer in addition to surgery. However, its benefit in survival remained obscure. This study was aimed to measure the efficacy of preoperative radiation therapy for survival in stage II and III rectal cancer patients. Retrospective cohort study used the database of Surveillance, Epidemiology and End Results program of the National Cancer Institute in the United States from 1988 to 2011. A total of 49,439 patients diagnosed with primary rectal cancer who underwent surgery were included. Clinicopathological characteristics and rectal cancer-specific survival between surgery alone group and surgery plus preoperative radiation therapy group were compared. Rectal cancer patients in surgery plus preoperative radiation therapy group had significantly better survival than those in surgery alone group (72.70% vs. 66.61%, p<0.001), as well as stratified by stages (stage II: 77.4% vs. 74.3%, p<0.001; stage III: 68.3% vs. 58.6%, p<0.001). However, this beneficial impact was only observed after 2000s (p<0.001). Multivariate survival analysis revealed that preoperative radiation therapy was an independent predictor for better survival in stage III (hazard ratio, 0.795; 95% CI, 0.753-0.840; p<0.001), but not in stage II (p=0.70). Preoperative radiation therapy might bring a better survival in stage II and III rectal cancer patients, but only as an independent predictor for stage III patients. As time progressed, preoperative radiation therapy might yield more profit for stage II and III rectal cancer patients.