PURPOSE: To investigate the role of STAT5 during osteogenesis differentiation of human mesenchymal stem cells(hMSC). MATERIALS AND METHODS: To assess the expression pattern of STATs during osteogenic differentiation, we performed western blot analysis and RT-PCR. By RNA interference, we have performed effect of STAT5A and STAT5B in osteogenesis. As a result of Luciferase assay, the promoter activity of DLX5 was decreased by STAT5A. RESULTS: To assess the expression pattern of STATs during osteogenic differentiation, we have performed western blot and RT-PCR after 14 day differentiation period. STAT1, 2, 3 and 4 showed no change in expression level during differentiation, while STAT5A and STAT5B displayed steady increase compared to the control. When STAT5A was knock down, the level of osteogenesis was increased. The transcriptional activity of DLX5 was decreased about 70% by STAT5A. CONCLUSION: The expression of STAT5A and STAT5B increased during osteogenesis of hMSC. Also, we shown that STAT5A regulated the transcriptional activity of DLX5. These results indicate that STAT5A acts as a pivotal transcription factor in osteogenesis of hMSC.