Abstract: Objective 　To investigate the anti2tumor effect of spleen tyrosine kinase (Syk) in vit ro and in vi vo. Methods 　Human breast cancer cell line MDA2MB2231 which expressing Syk negatively was cul2 tured in optimum medium and t ransfected with Syk cDNA by liposome, and cells t ransfected with blank vector were used as cont rol. The inhibited effect of Syk cDNA on the cell proliferation and the ability of assorting in vit ro was detected by MTT and Transwell2ECM methode ; the pulmonary metastasis and tumor growth of t ransformed and unt ransformed human breast cancer cells were investigated by xeno2 graf t s in BABL/ c (nu/ nu) athymic mice. Results 　There is no obviously change in proliferation in vit ro, between the MDA2MB2231 cells t ransformed Syk cDNA and t ransformed idling or not t ransformed, but number of the migrating cells (migrating cells/ 5 HPF) reduces obviously in invasion and immigration ex2 periment ( P < 0. 05) ; in the same, there is no difference in tumor growth of athymic mice inoculated dif2 ferent cancer cells, nevertheless, there is lower incidence of pulmonary metastasis in the group of inocula2 ted cancer cells t ransforming Syk cDNA ( P < 0. 05) . Conclusion 　Syk can rest rain in tumor by reducing the abilities of invasion and migration of breast cancer cells.