The purpose of this study was to identify the prognostic predictors treated with postoperative irradiation in patients with thymoma. Two hundred forty-one patients with histologically confirmed thymoma were collected and retrospectively reviewed in this study. Fifty-four patients had stage I disease; 57, stage II; 120, stage III; 10, stage IV. One hundred sixty patients underwent total thymectomy; 63, partial resection; 18, debulking or biopsy. Patients were irradiated after surgery with median dose of 50 Gy by conventional fractionation. The overall survival rates at 5 and 10 years were 83.1% and 72.6%, respectively. The 10-year overall survival was 87% for stage I, 78.7% for stage II, 57.4% for stage III, and 24.3% for stage IV. The conclusions were drawn from this analysis. For stage I, the role of postoperative irradiation needed further investigation. For stage II-III, surgery and postoperative irradiation should be part of standard care. The favorable prognostic predictors were female, early stage, and surgical extirpation.