In order to examine the presence of brassinosteroid substance in sea mustard(Undaria pinnatifida), leaves of sea mustard were extracted with MeOH. The extract was purified by slovent fractionation, counter current distribution, silica gel adsorption chromatography, charcoal adsorption chromatography, Bondesil chromatography, and reverse phase HPLC, successively. The activity was monitored by the rice inclination test and its prescence could be confirmed in each purification step. Although sea mustard contained a less amount of the active substance than the vegetative tissue of higher plants, brassinosteroid was clearly present endogenously in sea mustard. We acknowledge that our work is probably the first publication reporting the presence of brassinosteroid in marine algae plants.