Abstract： Objective To study the effect of osteopontin (OPN) expression down-regulated by RNA inter-ference(RNAi) on the proliferation and invasion of breast carcinoma cell line MCF-7. Methods MCF-7 cells were transfected with the chemically synthesized small interfering RNA (siRNA). RT-PCR and Western blotting were used to quantify the mRNA and OPN protein levels. The proliferation and invasion of MCF-7 were observed by cell growth curve and colony formation, experiments. Results After being transfected by sequence-specific siRNA targe-ting OPN, the expression of OPN RNA and protein decreased in MCF-7 cells. MTT test showed that the proliferation was inhibited significantly by siRNA compared with the control groups. The number of formed colonies was decreas ed in MCF-7 cells transfected with OPN - specific siRNA compared with control groups (P < 0.05). Condusion Sequence-specific siRNA targeting OPN can effectively inhibit the expression of OPN mRNA and protein levels, and significantly diminishes the proliferation and invasion of MCF-7 cells.