The antioxidant and prooxidant activities of six B group vitamins against hydroperoxide generation in linoleic acid peroxidation were analyzed by the aluminum chrolide method. The B group vitamins were divided into three types by their antioxidant and prooxidant properties. The first type, including B₁, B₂ vitamin, nicotinic acid and folic acid, showed prooxidant activities in the early phase (1 week) of the linoleic acid peroxidation at concentrations of 2.5 μM－2.5 mM, but caused relatively strong antioxidant activities in the later phase reaction (3 weeks) at the same concentrations. The second type, such as B₁₂ vitamin, did not cause significant effects in the early phases (1－2 weeks) of linoleic acid peroxidation, but exhibited significant antioxidant activities in the later phase reactions (3 weeks). The third type, such as B₆ vitamin, exhibited significant antioxidant activity in the early phase reaction and caused strong antioxidant effects in the later phase reaction (3 weeks). Generally, the antioxidant activities of B group vitamins in the later phases of lipid peroxidation was much stronger than those in the earlier phases. These experimental results suggest a possibility that B group vitamins have both antioxidant and prooxidant effects on lipid peroxidation under different experimental conditions.