Prostate cancer, a common malignant tumor of the genitourinary system among elderly males, is difficult to cure. Capilliposide C (CPS-C) is a novel oleanane triterpenoid saponin derived from Lysimachia capillipes Hemsl. Although CPS-C had been investigated in gastric cancer BGC-823 cells, prostate cancer PC3 cells, and ovarian cancer SK-OV-3 cells in nude mice in a dose-dependent manner without overt toxicity, its mechanism in the cancer cells has not been further studied. In the current study, PC3-cell-line-based in vitro models were used to explore the anti-cancer effect and mechanism of CPS-C. The results of cell viability assays showed that CPS-C exhibited dose- and time-dependent cytotoxicity against the prostate cancer cell lines PC3 and DU145. Observations using optical and electron microscope suggested the cytotoxicity of CPS-C. CPS-C-induced apoptosis was confirmed by the activation of caspases, down-regulation of Bcl-2, JNK, and P38α/β, and up-regulation of Bax, p-JNK, and p-P38. Results showed that CPS-C exhibits high toxicity toward two prostate cancer cells in vitro. Therefore, CPS-C may have great potential in the prevention and treatment of human prostate cancers.