Arsenic trioxide (As2O3) shows a significant therapeutic effect upon acute promyelocytic leukemia (APL) and can induce the apoptosis of NB4 cells, which attracts scholars' great attention. Especially, the therapeutic effect on solid carcinoma has been paid more close attention to. The present study is to evaluate the effect of As2O3 on human colorectal carcinoma cells (LS-174T cell) and the activity of telomerase in vitro and in vivo. This research made use of the electron microscope, polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA), fluorescence-activated cell sorter (FACS), MTT in vitro and in vivo (LS-174T xenograft model of nude mice). With the increasing concentration of As2O3, the ratio of living cells to dead cells decreased significantly, and the IC50 value was 5.23micromol/L; cells of the experimental groups endured a series of morphological changes similar to the features of apoptosis. Apoptosis curve of FACS pictures appeared after 24h, and the cells showed apoptosis in a time-dependent manner; As2O3 can inhibit the activity of telomerase of the cell extraction, obviously, in a concentration-dependent and time-dependent manner after 24h. As to the inhibition impact of As2O3 on the xenograft model of nude mice in the two indexes, tumor volume and weight, there was a significant difference between As2O3 and the control group; there was no difference between As2O3 and the fluorouracil (5-FU) group; in the group of peritoneal injections of As2O3, the cancer cells connected loosely with each other, nucleus changed markedly, and heterochromatin concentrated under the nucleus membrane. From the in vitro and in vivo experiment, we can see that As2O3 inhibited LS-174T cell growth mainly by inducing cell apoptosis, partly by the inhibition of telomerase activity.