Bioassay-guided fractionation of the aerial parts of Pilea mongolica(Urticaceae) afforded two cytotoxic triterpenoids, epi-oleanolic acid (I) and oxo-oleanolic acid (II). The structures of the compounds were confirmed by spectral and synthetic evidences. Compound I and compound II exhibited cytotoxicity against cultured human tumor cell lines, A549 (non small cell lung adenocarcinoma), SK-OV-3 (ovarian), SK-MEL-2 (skin melanoma), XF498 (CNS) and HCT15 (colon) with $ED_{50}$ values of $3.2-8.1{mu}g/ml$ and $0.7-6.8 {mu}g/ml$, respectively.