Objective To evaluate the risk factors of the patients with myasthenia gravis (MG) after resection of thymoma. Methods We retrospectively analyzed the clinical data of 126 thymoma patients without preoperative MG who underwent a thymectomy in our hospital from June 2002 through May 2015. There were 51 males and 75 females at age of 51.71±14.06 years. The risk factors for MG after resection of thymoma were evaluated. Results MG occurred in nine patients after resection of thymoma (7.1%). Incomplete resection (P=0.024), A and AB type of WHO classification (P=0.048), concomitant autoimmune diseases (P=0.024), postoperative pulmonary infection (P=0.036) were the risk factors for the MG after resection of thymoma. Postoperative radiotherapy and chemotherapy (P=0.011) reduced the risk for the patients with incomplete resection or invasive thymoma. Conclusion Incomplete resection, A and AB type of WHO classification, concomitant autoimmune diseases, postoperative pulmonary infection are considered as the risk factors for MG after resection of thymoma, and postoperative radiotherapy and chemotherapy should be performed for the patients with incomplete resection or invasive thymoma.