Shortening of the chromosome caused by DNA incompleted replication of chromosome end (telomeres) at each cell division leads the cells to lose proliferative capacity and senescence.Telomerase elongates telomeric DNA.And the unlimted cell proliferation needs telomerase activation. About 85% of malignacy express positive for telomerase.The formations and functions of telomeres and the effects of telomerase on the telomere synthessis are reviewed.The measuring of telomerase and the relationship between cancer cell and activation of telomerase are also introduced. The possibility of the cancer treatment by the telomerase inhibition is discussed.