Objective:To study the expressions and clinical significances of PCNA, EGFR, Bcl-2 and Bax in thymoma. Methods: The expressions of EGFR, PCNA, Bcl-2 and Bax in 46 cases of thymoma and 11 cases of normal thymus were detected with S-P immunohistochemistry. The results were analyzed with the pathologic indexes. Results: The positive rates of EGFR, Bcl-2 and Bax in normal thymus were 18. 2%, 9. 1%,and 18.2% respectively, while in thymoma were 71.7%, 41.3%, and 15.2% separately. The expression of EGFR in thymoma was significantly correlated with Masaoka staging and Levine classification. The survival rate of EGFR negative patients was significantly higher thanthat of EGFR positive patients (P＜0. 01). PCNA labelling index was significantly higher in thymoma as (4.00±1.87)% than in normal thymus, (2.68±0. 62)% , which was significantly correlated with Levine classification. The expression of Bcl-2 in thymoma was also significantly correlated with Levine classification. The survival rate of Bcl-2 negative patients was significantly higher than that of Bcl-2 positive patients (P＜0. 01). The expression of PCNA, EGFR, Bcl-2and Bax in thymoma had no correlation with histologic type and myasthenia gravis (MG) (P＞0. 05). Conclusion: The expression of EGFR may be involved in the occurrence and development of thymoma. EGFR can be regarded as a supplementary predictor for Masaoka staging so as to predict the progression accurately.The expression of Bcl-2 may contribute to the occurrence of thymic carcinoma and Bcl-2 can be used as a biomarker identifying thymic carcinoma.