Aim: Observing the Impact of arsenic trioxide (As2O3) on LS-174T cell (colorectal carcinoma cell lines) and the activity of telomerase. Methods: Using the methods of electron microscope (EMS), PCR-Elisa, flow cytometry (FCM) and MTT in vitro (cell culture). Results: (1) With the increasing of concentration of As2O3, the ratio of living cells decreased significantly, IC50=5.23μg/ml; (2) Impact of As2O3 on cell morphology: In experimental group, cells turned round, cell membrane shrunk and nucleus concentrated at karyotheca; (3) Apoptosis curve appeared after 24h, cells turned to apoptosis with time-dependent manner; (4) As2O3 inhibited the activity of telomerase of cell extraction obviously with concentration-dependent and time-dependent manner after 24 h, 48 h and 72 h respectively. Conclusion: From the experiment in vitro, we can see that As2O3 inhibit LS-174T cell growth mainly by inducing apoptosis, partly by the inhibition of telomerase activity.