Objective To investigate the mechanisms of oridonin inducing apoptosis on acute leukeamia NB4 cells and its mechanism. Methods NB4 cells in culture medium in vitro were given with different concentrations (8, 16, 24, and 32 μmol/L) of oridonin. The inhibitory rate of the cells was measured by MTT assay, cell apoptotic rate was detected by flow cytometry (FCM), morphology of apoptosis was observed by Hoechst 33258 fluorescence staining, DNA fragmentation was assayed by agarose gel electrophoresis, caspase-3 expression was detected by Western blotting, and caspase-3 activity was assayed with colorimetric assay kit before and after apoptosis occurred. Results Oridonin (over 16 μmol/L) could inhibit the growth of NB4 cells and cause apoptosis significantly, the suppression was both in a timeand dose-dependent manner. Marked changes of apoptosis including condensation of chromatin and nuclear fragmentation were observed very clearly by Hoechst 33258 fluorescence staining and a characteristic "ladder" of DNA fragments was elicited by agarose gel electrophoresis; Western blot analysis revealed that caspase-3 was activated by the loss of caspase-3 proenzyme (32 kDa) and the appearance of its 20 kDa subunit, and that along with the apoptotic process caspase-3 activity was increased concurrently. Conclusion Oridonin can induce apoptosis in NB4 cells via activation of caspase-3. These results will provide laboratory evidence for the clinical treatment of acute leukemia with oridonin.