Objective: To present the clinical experience in treating thymomas and thymomas with myasthenia gravis (MG). Methods: Between 1975 to 1997, 258 patients with thymoma were surgically treated. 168 cases of thymus tumor (TT, Group I) were compared to 90 cases of thymus tumor with myasthenia gravis (TTMG, Group II). Results: In group I, the average age was 38.4 years, the youngest was 4 years old. In 93.4%, the diameter of tumors was 5cm. 61.2% were in stages III and IV of clinical pathology. In group II, the average age was 46.2 with the youngest of 20 years old, the diameter of tumor under 3cm and 5cm was 34.6% and 65.6%. 55.2% was in stage I of clinical pathology. Conclusion: Many cases of thymoma in early stage received early treatment. The diagnostic standard of thymoma in early stage is: (1) the diameter of tumor is below 3cm. (2) In stage I of clinical pathology. The clinical characteristics of TTMG are : short history, the rapid progress, serious MG symptoms, and the high incidence of thymus-crisis.