Objective To investigate the inhibitory effect of harmine on osteosarcoma cell proliferation and apoptosis by down regulating cyclooxygenase-2 (COX-2) expression.Methods Human osteosarcoma cell line U2OS was cultured in vitro and randomly divided into control group,study group 1,study group 2 and study group 3.The cells were cultured in 0,5 μmol/L,10 μmol/L and 20 μmol/L concentration of harmine for 48 hours.Cell counting kit-8 (CCK-8) method and flow cytometry were used to detect cell viability and apoptosis.The expression level of COX-2,proliferation and apoptosis related proteins and mRNA were detected by Western blot and real-time quantitative polymerase chain reaction (RT-qPCR),respectively.Results After cultured with different concentrations of harmine for 12,24 hours and 48 hours,the cell viability of the three study groups were significantly lower than that of the control group (P ＜ 0.05),while that of the study groups 2 and 3 were significantly lower than that of the study group 1 (P ＜ 0.05).The apoptosis rate of the three study groups were significantly higher than that of the control group (P ＜0.05),while that of the two groups were significantly higher than that of the study group 1 (P ＜ 0.05).After 48 hours of culture,the levels of COX-2,cyclin D1,proliferating cell nuclear antigen (PCNA),B-cell lymphoma-2 (Bcl-2) protein and mRNA expression in study group 2 were significantly lower than those in control group,while the expression levels of cleaved caspase-3 and BCL2-Associated X (Bax) in study group 2 were significantly higher than those in control group (P ＜ 0.05).Conclusions Harmine can inhibit the proliferation and promote the apoptosis of osteosarcoma cells by inhibiting the expression of COX-2 and regulating the expression of cell cycle and apoptosis related protein.