A clinical study for thymectomy for 23 patients in Myasthenia Gravis was done between May, 1982 and July, 1989. at the department of Thoracic and Cardiovascular Surgery, College of Medicine, Han Yang University. Among Z3 patients, male to female ratio was 11: 12 and Age of onset was ranged from 15 years-old to 52 years-old. Previous symptom duration from diagnosis until operation was ranged from 2 months to 96 months, and Mean duration was 28.74 months. The severity of disease was classified by Osserman`s classification preoperatively. In histopathology of thymus, Thymic hyperplasia 7 cases, Thymoma 7 cases, Invasive thymoma 3 cases, Within Normal Limit 5 cases, and Thymic Atrophy 1 case. Correlation between preoperative symptom duration and postoperative clinical course was statistically significant. The shorter of preoperative symptom duration, The better of postoperative clinical course. Grading of postoperative course was classified by Papatestas in 1975. Follow-up for postoperative course was ranged from 3 months to 7 year and 4 months, Cases of complete remission 1 year postoperative period were 9 cases and cases of Improvement were 8 cases, So totally 17 cases among 23 cases[73.91%] found good results.