The aim of this study was to investigate the apoptotic effect and its mechanism on Radix Paeoniae Alba Extract(RPAE) in DU145 human prostate cancer cell line. RPAE induced apoptosis in a dose-dependent manner in DU145 cells as confirmed by both discontinuous DNA fragmentation using Hoechst33342 staining and poly-(ADP-ribose) polymerase(PARP) cleavage, which are apoptotic signs. To clarify the mechanisms on RPAE-induced apoptosis, we examined the p50(NF-κB subunit), IκBα, PTEN and Par-4 protein expression using Western blotting. Treatment with RPAE resulted in the decrease of p50 expression by IκBα increase, which resulted in Par-4 increase and bcl-2 decrease in DU145 cells. These results suggest that apoptosis of DU145 cells by RPAE involved decreases of NF-κB activation and bcl-2 expression, increase of Par-4 protein expression.