The antioxidant effect of marchantinquinone was studied using various models. Marchantinquinone inhibited Fe2+-induced lipid peroxidation in rat brain homogenate in a concentration-dependent manner with an IC50 15.3 ± 2.9 μM. Marchantinquinone was less effective than α-tocopherol and BHT in reducing the stable free radical diphenyl-2-picrylhydrazyl (DPPH). The IC0.200 were 37.5 ± 2.5, 9.2 ± 1.6 and 14.5 ± 2.5 μM, respectively. Marchantinquinone also scavenged peroxyl radical derived from 2,2′-azobis(2-amidinopropane)dihydrochloride (AAPH) in aqueous system with a stoichiometric factor of 0.9 ± 0.1. Furthermore, it prevented conjugated-diene formation and apolipoprotein B (apo B) oxidation of LDL. However, marchantinquinone did not chelate Fe2+ or possess Superoxide anion and hydroxyl radical scavenging activity. It also did not scavenge 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN)-derived peroxyl radical in hexane. These results indicate that marchantinquinone may be an effective antioxidant and can protect rat brain homogenate and LDL against oxidation.