Objective:To investigate the expression level of YY1 in gastric cancer,and then detect the effect of YY1 on the proliferation and apoptosis in SGC－7901 cell lines and its possible mechanism. Methods:mRNA and protein expressions of YY1 in cancer and paired adjacent tissues were detected by real－time PCR and immunohistochemistry,respectively. YY1 was up－regulated by a lentiviral vector expression system in gastric cancer cell line SGC－790. CCK－8 was performed to detect cell proliferation,and colony formation assay was performed to evaluate cell growth. Apoptosis was detected by flow cytometry. Results:The expression of YY1 in paired adjacent tissues had a higher level than that in tumor tissues. Compared to the empty vector group (SGC－7901－Empty Vector/SGC－7901－EV) and the normal control group (SGC－7901),the cell proliferation and clonogenicity were significantly decreased as well as cell apoptosis was increased in the over－expressed YY1 group (SGC－7901－YY1). Conclusion:YY1 may participate in the development of gastric cancer. YY1 can play a role as a cancer suppressor gene in the development of gastric cancer,which suggests that YY1 may be a new therapeutic target for gastric cancer.