Abstract: Objective:To evaluate the clinical value and the prognostic factors of postoperative radiotherapy in type B 3 thymoma pa-tients. Methods:A total of 159 patients with thymoma were treated by surgery and postoperative radiotherapy. According to Masaoka staging system, 12, 33, 62, and 52patients had stage Ⅰ, Ⅱ, Ⅲ, and Ⅳlesions, respectively. Myasthenia gravis existed in 38patients. Altogether 58patients underwent chemotherapy. Overall survival, disease-free survival, and local control rates were calculated by Ka -plan-Meier method. Prognostic factors were analyzed by Cox regression model. Results:With a median follow-up of52months (8- 125 months), the overall5-year survival rate was 81. 6%. The 5-year progression-free survival rate was 76. 2%. The 5-year local control rate was 82. 6%. The recurrence rate was32. 6%, and the metastatic rate was 9. 3%. In the univariate analysis, tumor size, Masaoka stage, re -section margin, radiotherapy, and chemotherapy were significantly associated with 5-year overall survival and progression-free surviv -al (P<0. 05). In the multivariate analysis, Masaoka stage, resection margin, and radiotherapy were independent prognostic factors of 5-year progression-free survival ( P<0. 05). Radiotherapy could improve the regional control rate and the overall survival of patients in Ma-saoka stages Ⅲ- Ⅳ. Conclusion: The major failure mode for type B3 thymoma is the recurrence of pleure. Radiotherapy can improve the regional control rate and the overall survival of patients in advanced stages. Masaoka staging, surgical margin, and radiotherapy are the independent prognostic factors for type B3 thymoma treated by postoperative radiotherapy.