The BuOH fraction prepared from the methanol extract of Acanthopanacis Cortex showed inhibitory activity against ADP-induced platelet aggregation. The inhibitory activity remained in ether layer when the BuOH fraction was refluxed with 5% aq. HCl-EtOH (1 : 1 mixture) and extracted with ether. From the ether layer, ethoxy-hydroxy-benzoic acid $(m.p.;128{sim}130^{circ}C)$, a platelet antiaggregating substance, was isolated.