Abstract: Aim ：To study the effects of Clostridium difficile toxin A on the cytotoxity and apoptosis of human hepatoma cell line SMMC-7721. Methods ：Clone inhibiting experiment and MTT calorimetric assay were used to assay SMMC-7721 proliferation. Morphological changes relevant to the apoptosis and the DNA damage were analyzed by transmission electron microscope and single cell gel assay. The number of apoptosis cells and the expression of Bcl-2 and p53 protein were detected by the flow cytometry. Results ：0.018- 4.690 mg/L toxin A significantly decreased the colony information of SMMC-7721 cells，and greatly inhibited SMMC-7721 proliferation in a time- and concentration-dependent manner. Morphological changes related to apoptosis were evident under transmission electron microscope and DNA damage was detected by single cell gel assay when SMMC-7721 cultured with 4.690 mg/L toxin A for 48 h. Toxin A 0.073-4.690 mg/L induced apoptosis in SMMC-7721 cells from 6.8% to 41.8%. In addition，Toxin A 0.293-4.690 mg/L significant decreased Bcl-2 protein expression and increased p53 protein expression in SMMC-7721. Conclusion ：These results showed that Clostridium difficile toxin A had a significantly cytotoxicity in human SMMC-7721 which was attributed to toxin A induced apoptosis.