 This is a randomized controlled clinical study. The investigators screen of eligible patients, randomized divide into the following two groups: corticosteroids azathioprine group, corticosteroids leflunomide group. The investigators treat the enrolled patients, estimate efficacy and observed the side effects according to the requirements of program. The investigators establish a clinical database for recording patients date and statistical analysis. Evaluation of short-term and long-term efficacy of thymectomized myasthenia gravis patients in the different group prove that what kind of treatment can improve the cure rate. The investigators will evaluate the acute toxicity gastrointestinal side effects, liver and kidney dysfunction and long-term toxicity immune dysfunction, gonadal suppression when the investigators apply these therapy in the treatment of different clinical types of MG. 