The expression of osteopontin (OPN) has been correlated with tumor growth and metastasis. However, the mechanisms by which OPN promotes tumor metastasis remain unclear. In this study, we aimed to investigate the anti-tumor effects of OPN by silencing OPN expression in the gastric cancer cell line SGC7901, using lentiviral-OPN small interfering RNA (siRNA) technology. Plasmid vectors containing OPN siRNAs were generated, encoded with lentiviral vector and transfected into SGC7901 cells (SGC-OPN- cells). OPN mRNA and protein expression were examined using real-time reverse transcription polymerase chain reaction (RT-PCR) and Western blotting techniques. The tumorigenicity and metastatic potential of SGC7901 cells were studied in nude mice. Expression of OPN and vascular endothelial growth factor (VEGF) in lung metastatic tumor specimens were also examined using immunohistochemistry (IHC). Among the three siRNA sequences tested, siRNA2 most remarkably inhibited mRNA levels of OPN; lentiviral-siRNA2 was stably transfected into SGC7901 cells to generate SGC-OPN- cells. SGC-OPN- cells had significantly decreased OPN expression compared to control cells (relative intensities were 0.14 ± 0.06 vs. 0.95 ± 0.16 in controls, P<0.01). A substantial reduction in detectable tumors was found in mice implanted with SGC-OPN- cells compared to controls (4.62 ± 1.24 vs. 8.35 ± 2.27 cm3 in controls, P<0.01). In addition, mice implanted with SGC-OPN- cells survived longer (101.2 ± 22.5 vs. 89.2 ± 24.6 d, P<0.01) and were demonstrated to have less metastases compared to mice implanted with SGC7901 control cells. Interestingly, lentiviral-siRNA2 also suppressed the expression of OPN and VEGF in metastatic lung specimens. Lentiviral-mediated OPN siRNA significantly reduced OPN gene expression, suppressing the growth and metastasis of gastric cancers, which might be related to reduced expression of VEGF. Therefore, OPN could serve as a promising therapeutic target for gastric cancer.