The therapeutic action of arsenic trioxide( As2O3 ) on solid tumors has aroused widespread interest among scholars. To study the impact of As2 O3 on human colorectal carcinoma cells( LS-174T cell) and the activity of telomerase,the methods of PCR-ELISA, flow cytometry (FCM) and MTT assay in vitro were utilized. The results show that ( 1 ) with an increase in the concentration of As2 O3, the ratio of living the cells to dead cells decreases significantly,and the IC50 value is 5. 23 μg/mL; (2) the cells of the experimental groups can endure a series of morphological changes similar to the features of apoptosis; (3) the apoptotic curves of FCM pictures appear after 24 h, and the cells show the apoptosis in a time-dependent manner; (4) As2O3 can inhibit the activity of telomerase of the cell extraction obviously in a concentration-dependent and time-dependent manner after 24 h. It can be concluded from the experiment results in vitro that As2O3 can induce the apoptosis of LS-174T cells and inhibit the telomerase activity. Therefore, it has been proposed, for the first time, that these two factors( the apoptosis of LS-174T cells and the inhibition to the telomerase activity) are important causes of the LS-174T cell death caused by As2O3.