p-Hydroxymethoxybenzobijuglone (HMBBJ), a new quinone compound isolated from Juglans mandshurica (by bioassay-guided fractionation), showed cytotoxic activity in the gastric carcinoma cell line BGC823. The growth of BGC823 cells was inhibited as demonstrated by MTT assay and several cellular characteristic changes, such as cell shrinkage, chromatin condensation and apoptotic body formation with programmed cell death. Flow cytometry analysis revealed that the BGC823 cell cycle was arrested at G2/M phase by HMBBJ, and the apoptotic rate of BGC823 cells increased with respect to HMBBJ in a dose-dependent manner. HMBBJ also activated caspase-3, decreased the expression of Bcl-2 and caused a decrease in the mitochondrial membrane potential (Delta Psi(m)). These findings suggest that HMBBJ could significantly induce apoptosis in BGC823 cells and should be considered as a potential candidate for a chemotherapeutic drug against cancer.