OBJECTIVE To discover the effect and mechanism of exogenous microRNA-29b (miR-29b) on proliferation, apoptosis and the sensitivity to chemotherapy of osteosarcoma (OS) cells. METHODS We assessed the expression of microRNA-29b in osteosarcoma tissues evaluating the regulation of in on the OS cell growth and drug sensitivity in human osteosarcoma MG-63 cell model. Firstly, quantitative RT-PCR (reverse transcription-PCR, RT-PCR) was used to measure the expression of miR-29b and matrix metallopeptidase 9(MMP-9) in primary osteosarcoma samples, and to evaluate the correlation between the two molecules. Secondly, miR-29b mimics or mimics were used to modify its expression in MG-63 cells. Luciferase reporter assay, Western blotting, cell viability, colony forming assay and apoptosis examination were performed to assess the regulation by manipulated miR-29b in the osteosarcoma-derived cells. RESULTS We found that miR-29b is down-expressed, whereas the MMP-9 level was markedly higher in primary osteosarcoma tissues and osteosarcoma-derived cells. We also found that exogenous miR-29b reduces the proliferation, promotes the apoptosis and upregulates the sensitivity to chemotherapy (doxorubicin) of osteosarcoma cells via direct targeting of the MMP-9. CONCLUSIONS Our data suggest that the reduced miRNA-29b may serve as a predictor of response to chemotherapy and as a therapeutic target in human osteosarcomas.