Abstract: Objective To investigate the tumor-suppressing effect of microRNA-218 (miR-218) in osteosarcoma (OS) and explore its molecular mechanism. MethodsWe examined the expression levels of miR-218 in 68 pairs of OS and adjacent tissue samples using qRT-PCR. Cultured human OS cell line Saos-2 was transfected with miR-218 mimics or anti-miR-218 mimics, and the cell apoptosis was assessed using CCK-8 assay, annexin V-FITC staining andWestern blotting.We also analyzed the potential functional targets of miR-218 in Saos-2 cells using luciferase assay, qRT-PCR andWestern blotting. Results The expression level of miR-218was lowered by at least 8 folds in OS tissues as compared with the adjacent tissues. In cultured Saos-2 cells, transfection with miR-218 mimics for 24, 36, and 48 h resulted in a significant reduction in the cell viability, while transfection with anti-miR-218 mimics significantly increased the cell viability. The cells transfected with miR-218 mimics showed an obviously enhanced expression of cleaved poly (ADP-ribose) polymerase (C-PARP) as compared with the cells transfected with anti-miR-218 mimics and the control cells. Flow cytometry demonstrated obviously increased apoptosis of the cells following miR-218 mimics transfection. We identified the oncogene B lymphoma mouse Moloney leukemia virus insertion region 1 (BMI-1) as a specific target of miR-218 in Saos-2 cells. BMI-1 expressions at both the mRNA and protein levels were significantly reduced in Saos-2 cells overexpressing miR-218 but increased in the cells with miR-218 knockdown as compared to the control cells. Luciferase reporter assay indicated that miR-218 directly inhibited the expression of BMI-1 via binding to its 3’-UTR in OS cells. Conclusion miR-218 can promote OS cell apoptosis and plays the role as a tumor suppressor by down-regulating BMI-1.