Objective To investigate the role of OPN in human breast cancer cell line ( MDA-MB-231) by using small interfering RNA to specifically knockdown OPN expression. Methods OPN ShRNA expression vector was stably transfected to MDA-MB-231 cell line.The expression of OPN mRNA and protein were analyzed using reverse transcription polymerase chain reaction (RT-PCR) and Western blot,respectively.The growth of MDA-MB-231 cells were observed by MTT.The effect of OPN siRNA on the transplanted tumor growth and tumor hypoxia were assessed in nude mice. Results The expression level of OPN in MDA-MB-231 cells were significantly lower under hypoxia or normoxia(P ＜ 0.05 ).OPN silence with RNAi significantly inhibited the invasion ability and proliferation of MDA-MB-231 cell lines (P ＜ 0.01 ).Inhibition of OPN with RNAi significantly inhibited the growth ability of MDA-MB-231 cells in vivo(P ＜0.05).The tumor hypoxia significantly decreased(P ＜ 0.05). Conclusions OPN silence with RNAi can effectively inhibit cell proliferation and tumor growth of MDA-MB-231 cells,and decrease the bypoxia level of MDA-MB-231 transplanted tumor in nude mice.