Abstract: Objective To study transfected shRNA of c-myc as therapeutic agent in target cell Colo320 which expressed more tolomerase activity and to investigate the effect of inhibition on telomerase activity and telomere lengths and tumor cell growth. Methods A plasmid-based polymerase III promoter system was used to deliver and express short interfering RNA (siRNA) targeting c-myc in Colo320 cells. The c-myc and hTERT mRNA levels were monitored by fluorescence real time reverse transcription-polymerase chain reaction, the protein levels of c-myc and hTERT were examined by Western blot analysis.Meanwhile, telomere lengths and telomerase activity were measured by Southern analysis of telomere restriction fragment (TRF) length and PCR-ELISA. We also assessed the effects of c-myc silencing on tumor growth by DNA synthesis （3H-thymidine）. Results Our data showed that expressions of c-myc and hTERT were decreased in shRNA-transfected cells, and down-regulations of c-myc and hTERT inhibited cell growth, reduced cell telomere lengths, telomerase activity. Conclusion shRNA of c-myc has the ability to inhibit telomerase activity, telomere lengths and cell growth with a dose dependent pattern.