Berberine has anti-tumor properties in some cancer cells including prostate cancer, but the exact mechanisms and in vivo effects are unclear. We investigated anti-cancer activity of berberine in vitro and in vivo, and possible mechanisms in prostate cancer cells. Berberine treatment inhibited cell cancer growth in a concentration (0-50 microM) and time- (0-48 h) dependent manner without any growth inhibition in normal human prostate epithelial PWR-1E cells. However, the p53 expressing LNCaP cells were more susceptible against berberine than the p53 lacking PC-3 cells. The cell arrest in G0/G1 phase, apoptotic cell death and the expression of apoptotic cell death proteins Bax and caspase-3 was much higher in berberine-treated LNCaP cells than those in PC-3 cells. Exploration of p53 siRNA or pifithrin-alpha, a p53 inhibitor to the LNCaP cells, suppressed berberine-induced cell death and expression of apoptosis-related proteins. In xenograft in vivo studies, berberine reduced tumor weights and volumes accompanied with apoptotic cell death and increased expression of apoptotic cell death proteins, however, the extent of inhibitory effect was more significant in LNCaP cell-bearing mice. Therefore, these results indicated that berberine inhibited p53-dependent prostate cancer cell death.