The effects of lipoxygenase, linoleic acid, tocopherol and pH on the co-oxidation of $eta$-carotene in the aqueous system were studied. It showed that the co-oxidation of $eta$-carotene was noticeable at both pH 7.4 and 9.0. As the concentraitons of linoleic acid and $eta$-carotene increased, the rate of oxidation of $eta$-carotene tended to be increased. However, $alpha$- and $delta$-tocopherol retarded the co-oxidation of $eta$-carotene. As the concentrations of tocopherols increased, $eta$-carotene was more stabilized, generally. But low concentration of $alpha$-tocopherol(10-4M) acted more effective antioxidant than high concentration of it(10-3M) at pH 7.4. The antioxidant effect of tocopherol greatly depended on pH ; it was outstanding at pH 7.4.