BACKGROUND: Tumour differentiation is an important index for adjuvant therapy in many cancers; however, non-small cell lung cancer (NSCLC) is an exception. Furthermore, postoperative radiotherapy (PORT) is controversial in patients with NSCLC with N0-1 and N2 disease. We aimed to evaluate the impact of tumour-related factors on overall survival (OS), cancer-specific survival (CSS), and distant control (DC) in patients with completely resected stage IIIA NSCLC. MATERIALS AND METHODS: Patients with stage IIIA non-metastatic NSCLC who underwent complete resection and adjuvant chemotherapy were identified from the Taiwan Cancer Registry (January 2007-December 2017). Logistic regression analysis was performed to determine the factors associated with PORT. Survival and relapse outcomes were compared using log-rank tests and Cox regression analysis. Sensitivity analysis was performed using propensity score-matched pairs. RESULTS: In total, 1,897 patients were included and stratified according to PORT use (PORT vs. non-PORT). After adjusting for covariates, PORT was not found to be associated with improved survival outcomes. In patients with poorly differentiated tumours and N2 disease, absolute benefits for OS (adjusted hazard ratio [aHR] 0.76), CSS (aHR 0.80), and DC (aHR 0.74) were observed. Multivariable hazard models of propensity score-matched pN2 disease and poorly differentiated tumour subgroups also showed significant survival benefit with PORT treatment. CONCLUSIONS: Patients with poorly differentiated tumours and receiving PORT for pN2 disease showed a lower risk of distant recurrence and more favourable survival outcomes in stage IIIA NSCLC with R0 resection.