Abstract: AimTo investigate the effect of oridonin (ORI) on telomerase activity and cell cycle of human leukemic cell line K562 cells. MethodsImmunohistochemistry (IHC) technique was used to determine the expression of hTERT or C-myc. Telomerase activity was detected with TRAP-PCR-ELISA assay. In addition, the percentages of K562 cells in different cell cycle were determined by flow cytometry (FCM) at 24th and 48th hours separately after adding the different concentrations of ORI. ResultsAfter the K562 cells were treated with ORI at 3.43 μmol·L^-1 for 48 h, the expression of hTERT and C-myc decreased obviously. There was statistical significant (P<0.05) difference between experimental groups and the normal controls. In addition, the telomerase activity of K562 cells was significantly inhibited by ORI at the dose of 3.43 μmol·L^-1 for 48 h. At the same time, the cell cycle distribution changed, the percentage of G₀/G₁ or G₂/M stages cells increased and that of the S stage cells decreased after ORI was added. Conclusion ORI can effectively inhibit telomerase activity in K562 cells. Arresting cell cycle and decreasing the expression of hTERT and C-myc may be the mechanism of action.