In order to search for the anti-HIV agents from natural products, eighty MeOH extracts of medicinal plants were applied to a syncytia formation inhibition assay which is based on the interaction between the HIV-1 envelop glycoprotein gp120/gp41 and the cellular membrane protein CD4 of T lymphocytes. Among them, Ailanthus altissima showed a potent virus-cell fusion inhibitory activity. Repeated column chromatoghaphy of the methylene chloride fraction of A. altissima afforded compoumds 1(β-sitosterol-3-O-β-D-glucoside), 2(tetramethoxycoumarin), and 3(ocotillone). Virus-cell fusion inhibitory activity of compound 3 (ocotillone) was 70.76±4.90% at the concentration of 100㎍/ml.