AIM: To investigate the effects of aspirin (acetylsalicylic acid) on proliferation and apoptosis of colorectal can- cer cell line $W480 and its mechanism. METHODS: Cyclooxygenase (COX)-2 negative colorec- tal cancer cell line SW480 was treated with aspirin at concentrations of 2.5 retool/L, 5.0 retool/L, 10.0 mmol/L for different periods in vitro. Anti-proliferation effect of aspirin on SW480 was detected by 3-(4,5-dimeth- ylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Cell cycle and apoptosis were observed by flow cytometry (FCM). Transmission electron microscope (TEM) was used for morphological study. Apoptosis-as- sociated genes were detected by immunohistochemical staining and West blotting. RESULTS: Aspirin inhibited SW480 proliferation and induced apoptosis in a dose- and time-dependent manner. Treatment with different concentrations of aspirin significantly increased the proportions of cells at the G0/G1 phase and decreased the proportions of cells at the S- and G2/M phases in a concentration- dependent manner. Aspirin not only induced apoptosis but also caused cell necrosis at a high concentration as well. After treatment with aspirin, SW480 cells displayed typically morphological features of apoptosis and necrosis under TEM, and increased the Bcl-2 expression in cells, but the expression of Bax was down regulated. CONCLUSION: Aspirin inhibits proliferation and induces apoptosis of SW480 cells. Its anti-tumor mechanism may arrest cell cycle and shift Bax/Bcl-2 balance in cells.