Abstract: Objective To study the role of ROS in the apoptosis of human breast cancer MCF-7 cells induced by valdecoxib, a selective COX-2 inhibitor. Methods MTT assay was used to observe the effect of valdecoxib on cell proliferation; Flow cytometry and hoechst 33258 dye were used to detect apoptosis; Laser confocal microscopy was used to detect the level of ROS and mitochondrial transmembrane potential; Western blot was used to detect the protein expression. Results 1）Valdecoxib significantly inhibited the proliferation of MCF-7 cells（P<0.05 or 0.01）and induced apoptosis of the cells（P<0.01）. 2）The expression of Bax was increased, and the expression of Bcl-2 was decreased after valdecoxib was apllied. The expression of caspase-3 was increased at first and then degradated to cleaved caspase-3. Caspase-3 inhibitor Ac-DEVD-CHO, and caspase inhibitor Z-VAD-FMK antagonized the growth inhibition effect of valdecoxib（P<0.05 or 0.01）. 3）Valdecoxib significantly decreased the mitochondrial transmembrane potential, and increased the level of ROS. Antioxidant NAC antagonized the growth inhibition effect of valdecoxib（P< 0.01）. Conclusion valdecoxib induces apoptosis of MCF-7 cells by increasing the level of ROS.

Key words: valdecoxib, cyclooxygenase-2, human breast cancer cell line MCF-7, apoptosis, ROS