It has been well known that apoptosis induction and cell cycle arrest are typical biological effects observed in cancer cells after proteasome inhibition. TH2 is a new natural xanthone analogue isolated from the resin of Garcinia hurburyi tree. Here, the cell growth inhibition of TH2 on human hepatocellular carcinoma cell line (Bel-7402) was evaluated in vitro using SRB assay. The treatment of 10 μmol/L TH2 reduced the surviving fraction from 86% (12 h) to 17.2% (48 h). To assess whether TH2 induce apoptosis, the appearance of sub-G1 peak, a specific fraction for apoptosis was detected by flow cytometry analysis. Progressive increase in the percentage of apoptotic population was observed in a dose-and time-dependent manner. Furthermore, a cleavage of poly (ADP-ribose) polymerase (PARP), a marker of early apoptosis, was observed clearly when the cells exposed to 10 μmol/L of TH2 for 24 h by immunoblotting analysis. In vitro activities of 20 S proteasome purified from human erythrocytes on fluorogenic peptide substrates revealed that TH2 inhibited the trypsin-like, chymotrypsin-like and peptidylglutamyl peptide hydrolyzing activities in dose-dependent manner. Moreover, the turnover of tumor suppressor p53, a sign of deregulation of cell cycle progression and apoptosis induction by classical proteasome inhibitors, was disrupted in Bel-7402 cells. All these data indicate that TH2 had inhibitory effect on the proliferation of Bel-7402 cells and induction of apoptosis, which might be related to its inhibition of proteasome.