We experenced 18 patients with surgically treated thymoma from January 1986 to December 1993. There were 13 male and 5 female ranged from 23 to 69 years of age. Among them Myasthenia gravis was present in 8 patients (44%) The predominant cell type was lymphocytic(11 patients), followed by epithelial (3) and mixed (4), and had no value in predicting prognosis. Treatment consisted of complete resection in 15 patients, partial resection in 2 patients and 1 patient was performed biopsy. Only, and then adjuvant radiation therapy was done in 7 patients and 3 patients needed adjuvant chemotherapy. Invasion of the adjacent tissue in thymoma was the most improtant prognostic value. There were 6 non-invasive tumors and 12 invasive tumors. Two patients with invasive thymomas resulted in death and one of 6 patients with non-invasive thymomas died during follow up ranged from 25 day to 60 months. The causes of death were myasthenic crisis in 1 patient, C. N. S. problem in 1 patient and pulmonary & mediastinal metastasis in 1 patient.