To explore the effect of antisense phosphorothioate oligodeoxynucleotide (ASODN) of human telomerase reverse transcriptase (hTERT) gene on telomerase activity in CEM cells, PCR enzyme-linked immunoassay was used to determine telomerase activity. The expression levels of hTERT mRNA and protein were assayed by RT-PCR and immunofluorescence assay using fluoresce isothiocyanate label respectively. The results showed that the expression levels of hTERT mRNA and protein in CEM cells decreased with time after hTERT ASODN treatment. There was no difference in hTERT mRNA and protein levels between control and sense oligodeoxynucleotide-treated cells. Telomerase activity decreased when CEM cells were treated with ASODN for 48 hours. Telomerase activity of CEM cells was significantly inhibited when treated with ASODN for 72 hours. There was no difference in telomerase activity levels between control and hTERT sense oligodeoxynucleotide-treated cells. These results suggested that hTERT ASODN inhibited telomerase activity of CEM cells.