Lipoxygenase which catalyzes the hydroperoxidation of the unsaturated fatty acids containing, cis-1, 4-pentadiene system was prepared as a partially purified form from the water-soluble extracts of rice bran, and its molecular and catalytic properties as a direct dioxygenase were studied by polarographic and sepctrophotometric methods. The enzyme showed a reasonable stability under the normal assay condition, but its activity was reduced markedly in the presence of chelating agents. When the initial velocity of the enzymic reaction was measured by the rate of oxygen uptake in the reaction mixture containing varying concentrations of linolate and molecular oxygen, the corresponding reciprocal plote showed non.parallel kinetic pattern. Such a steady state kinetic analysis along with a set of the esr study may suggest an involvement of a ternary complex between the enzyme and two ligand substrates during the hydroperoxidation reaction in which linolate may act as a leading substrate. The K_m values for linolate and the molecular oxygen were estimated to be about 118 and 100 μM, respectively. In addition, based on the kinetic pH-rate profile and functional group modification experiments, a probable mode of lypoxygenase catalyzed reaction was suggested.