ObjectiveTo investigate the long-term efficacy and the influencing factors of thymectomy for thymoma patients associated with myasthenia gravis. MethodsWe retrospectively analyzed the clinical and follow-up data of 126 thymoma patients associated with myasthenia gravis underwent extended thymectomy from June 2002 to December 2015 in our hospital. There were 26 males and 37 females at the mean age of 54.51±12.62 years. We built up survival analysis model to analyze the effect of those following factors on postoperative result:sex, the age when operated, the preoperative course of disease, the condition of associating with other diseases, history of critical illness, steroid administration time before operation, Osserman classification, Masaoka staging, WHO pathological type, surgical approach, tumor size and so on. Result The average follow-up time was 35(5-96) months. During follow-up period, 12 patients (19%) achieved complete remission, 39 patients (62%) achieved partial remission, 7 patients (11%) kept stable, 5 patients (8%) deteriorated and the total effective rate was 81%. The result of log-rank analysis showed that the preoperative course of disease (P=0.027), history of critical illness on myasthenia gravis (P=0.035) and Osserman classification (P=0.018) were related to incomplete remission, whlie the result of Cox regression analysis showed that the preoperative course of disease (P=0.001) and Osserman classification (P=0.012) were the independent risk factors for incomplete remission. ConclusionExtended thymectomy is an effective treatment for thymoma patients associated with myasthenia gravis, but the symptom of those patients whose preoperative course of disease are more than 12 months or Osserman classification is at ⅡB, Ⅲ and Ⅳ type of Osserman classification have poor effect after operation.