Serine threonine kinase 15 (STK15, also named BTAK, Aurora-A, aurora-2, or AIKI) is a type of mitotic kinase. The overexpression of STK15 is significantly associated with carcinogenesis in many tumors. The purpose of the present study was to investigate the expression of STK15 in lung squamous cell carcinoma and adenocarcinoma and analyze the correlation between STK15 expression and clinicopathological factors. The expression patterns of STK15 were examined by immunohistochemistry in 80 lung squamous cell carcinomas and adenocarcinomas and 20 normal lung tissues. The protein and mRNA expression of STK15 were evaluated by western blot and reverse transcription-polymerase chain reaction (RT-PCR) in 40 lung cancer samples and corresponding normal lung tissues. Immunohistochemically, the positivity of STK15 expression was 68.75% (55/80). The STK15 expression was significantly higher in poorly differentiated lung cancers than in well-differentiated or moderately differentiated lung cancers (P = 0.011). Western blot and RT-PCR showed that the protein and mRNA expression of STK15 were correlated (P = 0.044) and significantly higher in tumors than in corresponding normal lung tissues (P < 0.05). These results indicate that the overexpression of STK15 contributes to the carcinogenesis and de-differentiation of lung cancers.