Objective: To study the effects of genistein on the proliferation and cell cycle progression of human gastric carcinoma cells. Methods: 3H TdR incorporation test was used to investigate the cell proliferation. Flow cytometry was used to analyze the cell cycle arrest. Immunocytochemistry technique and Western blotting were used to observe the cyclin B and P21 waf1/cip1 protein expression. Results: Genistein inhibited the proliferation of tumor cells significantly, arrested cell cycle progression at G 2/M phase, and enhanced cyclin B and P21 waf1/cip1 protein expression in dose dependent manner. Conclusion:Proliferatory inhibition and G 2/M arrest of human gastric carcinoma cells after treated with genistein may be due to increased stability of cyclin B protein and the expression of P21 waf1/cip1 .