The present invention relates to a novel compound having excellent antioxidant activity, isolated from Ramalina terebrata, and more particularly to a novel compound, Ramalin, having excellent antioxidant activity, isolated from the Antarctic lichen Ramalina terebrata, a method for preparing the Ramalin, and a pharmaceutical composition, a functional food and a functional cosmetic composition, which contain the Ramalin as an active ingredient. The Ramalin according to the invention has significantly excellent antioxidant effects compared to commercially available antioxidants, and thus can be widely used in agents for treating oxidation-related diseases, anti-aging functional foods, functional cosmetic products for skin whitening and wrinkle reduction, etc.