OBJECTIVE To study the effect of hypoxia on the proliferation of human bone marrow mesenchymal stem cells (hBMSCs). METHODS The method of density gradient centrifugation was employed to isolate and culture hBMSCs. And flow cytometry (FCM) was employed to detect the cell surface marker. After establishing the experimental model of CoCl2-chemical hypoxia, MTT method and flow cytometry were applied to evaluate the proliferation and the proliferation index of hBMSCs at different time points with various CoCl2 concentrations. RESULTS The proliferations of hBMSCs was inhibited within the first 12 hours under chemical hypoxia condition. Compared with the normal group, the hBMSCs of each CoCl2 group were remarkably proliferated 1, 2, 4 days after chemical hypoxia with CoCl2, but 300 µmol/L CoCl2 group showed no significant difference (P > 0.05). 100 µmol/L CoCl2 group (0.139 ± 0.003, 0.178 ± 0.005, 0.224 ± 0.005) and 150 µmol/L CoCl2 group (0.202 ± 0.020, 0.224 ± 0.019, 0.263 ± 0.004) proliferation was significantly higher than that of the control group (0.134 ± 0.005, 0.167 ± 0.004, 0.206 ± 0.005). Compared with the normal group, the hBMSCs were remarkably proliferated 24 hours after chemical hypoxia with CoCl2 concentration of 150 µmol/L (all P < 0.05). At Day 6, the 100, 150 µmol/L CoCl2 group (0.258 ± 0.020, 0.264 ± 0.008) cells was still higher than that of normal group (0.248 ± 0.004), but the advantage gradually diminished (P < 0.05). At Day 7, the proliferative effects of hypoxia disappeared. CONCLUSIONS CoCl2-induced chemical hypoxia may promote the proliferation of hBMSCs.