Abstract: ObjectiveTo investigate the effect of Radix Clematidis Saponins(RCS) on NB4 cells apoptosis and its possible mechanism. Methods NB4 cells were treated with different concentrations of Radix Clematidis Saponins, using 0.01% DMSO as the negative control group，1.0 μmol/L Arsenic Trioxide as the positive control group.MTT method was used to detect the inhibition rate.The apoptostic rate and the alteration of cell cycles were assessed by flow cytometry determination and cell morphology was observed by wright's staining. Real-time PCR was used to evaluate the effects of sRadix Clematidis Saponins on the PML-RARa mRNA. Results Radix Clematidis Saponins was able to inhabit the NB4 cell growth, in time or concentration-dependent manner, but the inhibition rate was less than Arsenic Trioxide group. Wright's staining showed typical apoptotic morphological changes in Radix Clematidis Saponins group, the same as Arsenic Trioxide group. Apoptotic rate was increased in time dependent manner. The number of G2 stage cells was increased, while G1 stage was decreased (P<0.05). There was no difference of expression of PML-RARa mRNA between two groups or between different time periods of Radix Clematidis Saponins treatment. Conclusion Radix Clematidis Saponins might induce cell apoptosis, possibly by cell cycle arresting, but not reduce the PML-RARa mRNA gene expression.