Resveratrol has been shown to have antineoplastic effects in vivo and in vitro. However, the effect of resveratrol on the hypoxia-enhanced proliferation and invasion of osteosarcoma cells remains unclear. In this study, we investigated the role of resveratrol on regulating proliferation and invasion of osteosarcoma cells under hypoxic conditions. Saos-2 cells were cultured under controlled hypoxic conditions (3% O-2) or normoxic conditions. Resveratrol (50 mu M) was added in the medium; and hypoxia inducible factor-1 alpha (HIF-1 alpha) siRNA was used to inhibit HIF-1 alpha transcription. Proliferation of Saos-2 cells was evaluated by the methabenzthiazuron (MTT) assay. The invasive ability of Saos-2 cells was determined by a Transwell assay. HIF-1 alpha, E-cadherin and vimentin protein levels were detected by western blot analysis. HIF-1 alpha, E-cadherin and vimentin mRNA levels were assessed by RT-PCR. Compared to the control group, hypoxia significantly increased the proliferation rate and invasive ability of Saos-2 cells. Moreover, hypoxia markedly increased the E-cadherin level and decreased vimentin expression. However, resveratrol or HIF-1 alpha silencing reverted all the above effects of hypoxia in Saos-2 cells. Moreover, resveratrol inhibited HIF-1 alpha protein accumulation without affecting the HIF-1 alpha mRNA level. These data suggest that resveratrol can inhibit the hypoxia-enhanced proliferation, invasion and epithelial to mesenchymal transition process in osteosarcoma via downregulation of the HIF-1 alpha protein. Thus, HIF-1 alpha may be a promising drug target of resveratrol in the context of development of anticancer therapy for human osteosarcoma.