To investigate the anti-proliferation effect of oridonin on leukemic NB4 cells and its mechanism. NB4 cells in culture medium pan class="a-plus-plus emphasis">in vitro were given different concentrations of oridonin. The inhibitory rate of the cells were measured by MTT assay, cell apoptotic rate was detected by flow cytometry (FCM), morphology of cell apoptosis was observed by hoechst 33258 fluorescence staining, and the activity of telomerase was detected using TRAP-PCR-ELISA before and after apoptosis occurred. Oridonin (over 8 μmol/L) could decrease the telomerase activity, inhibit the growth of NB4 cells and induce apoptosis significantly in a time- and dose-dependent manner. Marked morphological changes of cell apoptosis were observed by hoechst 33258 fluorescence staining especially after the cells treated by oridonin for 48-0 h. Oridonin could inhibit the proliferation and induce the apoptosis of NB4 cells pan class="a-plus-plus emphasis">in vitro . One of the mechanisms may be the decrease of the telomerase activity of NB4 cells.