Incidence of hepatocellular carcinoma (HCC) is dramatically increasing and is the third cause of cancer death worldwide. One key approach to control HCC is chemoprevention by naturally occurring agents. This study aims at investigating the antitumor effect of oleanolic acid (OA) and the molecular mechanisms. BALB/c mice were injected subcutaneously with HepG2 cells to establish transplanted tumors. Apoptosis and cell cycle arrest-related markers and signaling cascades were determined by western blot, immunofluorescence, reverse transcriptase-polymerase chain reaction and flow cytometric analysis. OA exhibited inhibitory effect on HCC through induction of apoptosis and cell cycle arrest both in transplanted tumors and in HepG2 cells. OA induced apoptosis through mitochondrial pathway, evidenced by inhibition of Akt/mammalian target of rapamycin pathway, mitochondrial dysfunction, transient increase of adenosine triphosphate, increase of Bax/Bcl-2 ratio, increased release of cytochrome c and activation of caspase/poly (ADP-ribose) polymerase. Activation of mitochondrial apoptotic pathway may be due to reactive oxygen species generated by mitochondrial fatty acid oxidation, resulted from enhancement of lipolysis regulated by cyclic adenosine 3',5'-monophosphate response element-binding protein-hormone-sensitive lipase/peroxisome proliferator-activated receptor γ signaling. OA induced G2/M cell cycle arrest through p21-mediated downregulation of cyclin B1/cdc2. Cyclooxygenase-2 (COX-2) and p53 were involved in OA-exerted effect, and extracellular signal-regulated kinase-p53 signaling played a central role in OA-activated cascades responsible for apoptosis and cell cycle arrest. OA demonstrated significant antitumor activities in HCC in vivo and in vitro models. These data provide new insights into the mechanisms underlying the antitumor effect of OA.