Purpose: The aim of our study was to evaluate the impact of postoperative radiotherapy on the survival of lung cancer patients diagnosed with advanced carcinoma stages. Methods: We carried out a retrospective cohort study that consists of patients diagnosed with lung cancer from SEER database. The total analytical sample number of the study (n=130,291) was divided into two groups in regards to receiving Postoperative radiotherapy (PORT); intervention group (n=12,411) who received PORT and control group (n=117,880) who did not receive PORT. Chi-square test and Kaplan Meier method were used for performing the statistical analysis. Results: Observed survival rate of the intervention group, estimated by Kaplan-Meier method was found to be (76%, CI 95%), (65%, CI 95%), and (39%, CI 95%) for stages IIIA, IIIB and IV lung cancer respectively in comparison to the survival rates for patients with the same stages in the control group (55%, CI 95%), (45%, CI 95%), and (24%, CI 95%), respectively. Conclusion: Significant positive impact of postoperative adjuvant radiotherapy was noted on the overall survival of patients diagnosed with advanced lung cancer stages IIIA, IIIB and IV (p<0.05).