Abstract: Objective: To study the inhibitory effects of phosphorothioate-modified antisense oligodeoxynucleotides (PS-ODN) in LS-174T colorectal cancer cells and the mechanism of telomerase inhibition. Methods: The PS-ODN was used to infect the LS-174T cells and to block human telomerase RNA (hTR) using anti-sense technology, leading to induction of apoptosis in the LS-174T cells. The inhibitory action of PS-ODN was evaluated by means of electron microscopy (EMS), polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA) and flowcytometry (FCM). Results: PS-ODN showed a dose and time-dependent inhibition of cell proliferation. The optimal dose of PSODN was 10 μmol/L. Ten days after the treatment, the PS-ODN group exhibited a significantly different level of cell proliferation compared to the control groups (P<0.01). The results of PCR-ELISA indicated that telomerase activity was markedly inhibited in the PS-ODN group (P<0.01). A change in cellular number and morphology was also observed. The cells in the PS-ODN group showed apoptotic peak 72 hours after treatment, and this phenomenon did not occur in the controls. Conclusion: PSODN can inhibit telomerase activity and induce apoptosis. This treatment strategy has potential for treatment of malignant tumors.