Abstract: Objective To study the effect of Oleanolic Acid induced apoptosis in NB4cells in vitro.Methods CASY analyzer was used to evaluate the inhibitory effect of Oleanolic Acid on NB4 cells in vitro;DAPI staining was used to detect the morphological changes of nuclei of NB4 cells;Flow Cytometry analysis and argarose gel electrophoresis was used to evalutate apoptosis of NB4 and detect the chromatin DNA fragmentation.Results CASY assay analyzer showed that Oleanolic Acid dramatically inhibited the growth of NB4 cells in vitro;DNA Ladder was detected after exposure of NB4 cells to Oleanolic Acid;FACS analysis showed that Oleanolic Acid can cause NB4 cells was apoptosis;morphological changes of nuclei of NB4 cells were detected by DAPI staining.Conclusion These findings and data suggest that Oleanolic Acid induces NB4 apoptosis and inhibites growth of NB4 cells.