The hypoxia-inducible factor (HIF) 1alpha is a key regulator of the cellular response to oxygen deprivation. Specific disruption of the HIF-1 pathway is important for exploring its role in tumor biology and developing more efficient weapons to treat cancer. In this study, we stably transfected human breast tumor MCF-7 cells with short hairpin RNA expression vectors targeting HIF-1alpha. After knockdown of HIF-1alpha, hypoxia-induced expression of its target genes such as vascular endothelial growth factor, Glut-1, phosphoglycerate kinase, and P-glycoprotein were markedly attenuated. Moreover, HIF-1alpha knockdown was found to suppress the shift from S-phase to G(1) induced by hypoxia and increase drug sensitivity to methotrexate. The growth rates of HIF1alpha-knockdown tumors were drastically retarded in both subcutaneous and orthotopic xenograft models, which were accompanied by decreased angiogenesis and reduced expression of glucose transporter in tissue sections. These data demonstrate that HIF-1alpha knockdown reduces tumorigenicity of MCF-7 cells and suggest a promising combination of both anti-HIF-1 strategy and traditional chemotherapy to improve cancer treatment.