The methanol extracts of AraIia 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, $^1H-NMR;and;^{13}C-NMR$.