A novel antioxidant capacity assay for lipophilic compounds was developed using electron paramagnetic resonance (EPR) spectroscopy. The assay is based on antioxidant's scavenging ability against the tert-butoxyl radical generated photolytically from di-tert-butyl peroxide in ethyl acetate, and named the tert-butoxyl-based antioxidant capacity (BAC) assay. The radical was trapped by spin trap, 5,5-dimethyl-1-pyrroline-N-oxide, and EPR signal intensity of the spin adduct was used as a quantitative marker of radical levels. Signal intensity decreased in a dose-dependent manner in the presence of an antioxidant that competitively reacts with the radical, which was utilized to evaluate BAC values. The BAC method enabled the accurate estimation of antioxidant capacity for lipophilic materials that may counteract lipid peroxidation in biological membranes. The BAC values for quercetin and caffeic acid are 0.639 ± 0.020 and 0.118 ± 0.012 trolox equivalents, respectively, which are much smaller than values obtained by other aqueous methods such as H-ORAC and ORAC-EPR. Thus, antioxidants present in a non-aqueous environment should be evaluated using a non-aqueous system. In combination with in situ ascorbate reduction, the BAC method was capable of accurately determining the antioxidant capacity of water-insoluble materials that may be reduced in living cells.