Objective:To investigate the mechanism of nimesulide on human gastric carcinoma SGC-7901 cells apoptosis. Methods:SGC-7901 cells were incubated in RPMI 1640 containing different concentration of nimesulide(0,12.5,50.0,100.0,200.0,400.0 μmol/L). After treated with nimesulide,the cytologic morphological changes were observed by the optical microscope and transmission electron microscopy. The Power VisionTM two step method was used for the immunostaining of COX-2,bcl-2,bax. Westernblot analysis was employed to investigate the expression of COX-2,bcl-2,bax. Results:Through HE staining,the cells in negative control groups had normal structures,but with the increase of dose of nimesulide,the SGC-7901cells became sparse. In high concentration group,the dead cells showed morphologic changes of apoptosis,such as nuclear condenstion,fragmentation and chromatin crescent. By electron microscopy,cells in high concentration group also had apoptotic characteristics including chromatin condensation,nucleus fragmentation,chromatin crescent and apoptosis body. With the increase of concentration of nimesulide,the expressions of COX-2,bcl-2 decreased,while the level of bax protein increased by immunohistochemical and Western blot assay. There was positive correlation between the expression of COX-2 and bcl-2(rs=0.885 9;P < 0.001). Conclusion:Nimesulide can induces apoptosis,which may be related to overexpression of bax and inhibition of bcl-2.