The methanol extracts of Aralia elata showed antimicrobial activity against bacteria. The antimicrobial active principle was successively purified with solvent fractionation, silica gel adsorption column chromatography, Sephadex LH-20 column chromatography, silica gel partition column chromatography and HPLC. The active substance was isolated by HPLC on C_(18) column with an acetic acid-MeOH system. The active substance was identified as trans-4-hydroxycinnamic acid by MS, ¹H-NMR and ^(13)C-NMR.