Telomerase has been detected in more than 85% of human tumor cell lines and tissure, but not exist in majority of nomal cell and tissure. PCR and TRAP were employed in this experiment to test the telomerase activity changing of tumor cell line Hela, A431 and KB after irradiated.The result indicated that:tumor telomerase activity and expression of hEST2 functional area fragment are dose-dependent decreased;the result of PCR test method are corrlated with that of TRAP, those data suggest that the supression effect of irradiation onto tumor telomerase activity is one of critical mechanism of radiotherapy.Determining the telomerase activity level perhapes is a new index to evaluate the killing effect of irradiation onto tumor.