A new triterpenoid, 20(R),22$({zeta})$,24(S)-dammar-25(26)-ene-$3{eta},6{alpha},12{eta}$,20,22,24-hexanol (1), and three known triterpenoids, ${eta}$-D-glucopyranoside,($3{eta},12{eta}$)-12,20-dihydroxydammar-24-en-3-yl,6-acetate (2), 20(R)-ginsenoside $Rg_3$ (3), and 20(R)-ginsenoside $Rg_2$ (4), were isolated from the leaves of Panax ginseng. Their structures were determined by chemical analysis and spectral methods (IR, 1D and 2D NMR, HR-ESI-MS). Compounds 1-4 were exhibited various degrees of cytotoxicity in the human hepatoma cell line, HepG2. Compound 1 had the highest cytotoxic Potency, with an $IC_{50}$ value of 20.1 ${mu}M$, by stimulating p53-mediated cell cycle arrest at the G1 to S phase transition, leading to apoptosis via activation of the caspase signaling pathway.