Objective To investigate the effect of overexpression of cartilage oligomeric matrix protein (COMP) on BMP-2 induced cell osteogenic and chondrogenic differentiation of mesenchymal stem cells (MSCs).Methods MSCs,transfected with plasmid DNA encoding recombinant human COMP,were induced to differentiate into osteocytes and chondrocytes by BMP-2.Realtime PCR of osteogenic related markers (Col1a1,RUNX2,OPN,BGP) and chondrogenic related markers (Col2a1,SOX9,Aggrecan) were performed to evaluate the process of cell differentiation.ALP staining,Alizarin red S staining for osteogenic differentiation and alcian blue staining for chandrogenic differentiation were conducted to evaluate the tendency of cell differentiation.Results Real-time PCR assay presented the significantly higher (P＜0.05) COMP expression of MSCs when COMP gene was transfected into cells.The expression level of OPN was significantly (P＜0.05) down-regulated at all the time points in experimental group compared with that in control group.A final significant (P＜0.05) up-regulation of expression appeared in experimental group at the late stage of induction (day 7,14) compared with that in control group,even though a decrease (P＜0.05) expression of Col1a1,RUNX2 and BGP in experimental group occurred at the early stage of induction (day 3).The expression of Aggrecan and Col2a1 in experimental group was up-regulated (P＜0.05) at different time points compared with that in control group.And a significant higher (P＜0.05) expression of SOX9 in experimental group only appeared at day 7 compared with that in control group.ALP staining and Alizarin red S staining were weakened while alcian blue staining was enhanced.Conclusion COMP may inhibit BMP-2 induced osteogenic differentiation and promote BMP-2 induced chondrogenic differentiation,which may provide new insight for cartilage tissue engineering.