The present invention relates to: a cosmetic composition including a physiologically active antioxidant such as vitamin C or the like； a double-layer cosmetic including same； and a method for inhibiting oxidation of the antioxidant, wherein oxidation of the physiologically active antioxidant can be inhibited by covering a surface layer of an aqueous phase containing the physiologically active antioxidant with an oil phase to prevent exposure to oxygen in the atmosphere, and the process thereof is not complicated and thus has the effect of being efficient. In addition, the method for inhibiting oxidation of a cosmetic composition including a physiologically active antioxidant according to the present invention has an advantage in that the method can be used together with existing techniques, such as a technique for chemically stabilizing the physiologically active antioxidant, and a encapsulation technique by which the physiologically active antioxidant is stabilized by being trapped inside a capsule.
