Abstract: Objective:To study expression of STK33protein in lung cancer, mammary gland cancer, and colon cancer tissue and its relation to clinical stage. Methods:Tests were conducted on174 lung cancer, mammary gland cancer, and co -lon cancer tissue samples and examined together with additional patient management data from January 2007 to April 2009. Immunohistochemical technique and H&E staining were used to observe expression of STK 33protein in the lung cancer, mammary gland cancer, and colon cancer tissue samples. Results: STK 33lost its expression in 20benign lesion of lung and normal lungs, the expression of STK 33was 79.7% and 100 % in lung cancers (51/64) and lung cancer carcinoma -tous metastasis lymphoid tissue (16/16). There was a significant difference of expressionin STK33levels in lung cancer and lymphaden metastasis compared with normal lung tissue and lung benign lesions (P<0.01). The expression status of STK 33in lung cancers was associated with the lung cancer tumor cell differentiation and the lymphaden metastasis (P<0.05). The expression of STK 33in mammary gland cancers and colon cancers were significantly different to non-cancer -ous tissue (P<0.05). Western blot analysis demonstrated higher expression of STK 33in lung cancers than in non-cancer -ous tissue. Conclusion:A high expression of STK33is related with malignant tumors in lung cancer, mammary gland cancer and colon cancer and can be used as a reference sign for the diagnosis of pathological changes in the lung, mammary gland, and colon. As a tumor develops, the expression level of STK33tends to increase, thus raised expression of STK 33 protein indicates malignant phenotype and malignant behavior. This implies that STK33protein could become a molecule marker for post-operative evaluation for the prognosis of non-small cell lung cancer patients.