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 myasthenia gravis.

Inclusion Criteria:

1. 12 to 65 years；

2. Myasthenia gravis:

   1. Patients who are diagnosed as generalized or ocular myasthenia gravis
   2. have experienced extended thymectomy (including thymic hyperplasia and thymoma)， no significant complications in 6 months after operation , and does not received any immunosuppressants or glucocorticoids treatments.
   3. do not applied in plasmapheresis or immunoglobulins treatment during 3 months .
   4. women of child-bearing period do not have a plan of pregnant for at least 3 year.
   5. Written consent of the patient, after informing

Exclusion Criteria:

1. The liver , kidney or glycometabolic function is abnormal
2. Seriously complications, such as infection or symptom in central nervous system,
3. The patients who suffering from malignancy or a history of malignancy, a variety of sexually transmitted diseases and HIV infection, tuberculosis infection, and other condition which need to prohibit the use of immunosuppressive patients.
4. Be allergic to leflunomide, azathioprine
5. Pregnant or suckling period woman
6. Accompanied with mental disorders and have difficult to communication
7. Experienced myasthenia crisis in 3 months.
8. suffering from clear cardiopulmonary functional and brain abnormalities
9. Have a history of refractory hypertension or peptic ulcer .
10. One of the white blood cells, hemoglobin, and platelet count obvious abnormalities