Abstract: The esterification of decanoic acid with L-ascorbic acid in the presence of tert-butanol acid as the solvent and immobilized lipase (Novozym誖435) as the catalyst by means of ultrasonic irradiation to obtain L-ascorbyl decanoate (L-AD) was studied. By using ultrasound the dissolution rate of the reactants could be accelerated greatly, the reaction time of esterification could be reduced from 48 to 4h. The optimal actual yield was 78.48%.The purity of target product was above 98.6% after recrystallized. The immobilized lipase could be reused 4 times in ultrasonic field. In antioxidant activities of L-ascorbyl decanoate were studied by the items of hydroxyl radical system, system of DPPH·, and reducing power. Its antioxidant effect were further evaluated by adding into the soybean and sunflower oil. The result indicated that L -ascorbyl decanoate within the given concentration range in dose-effect relationship showed good antioxidant activities on the clearance of hydroxyl radical, superoxide anion free radical and the reducing power of DPPH·. Furthermore, antioxidant activities of L-ascorbyl decanoate were comparable with that of L-ascorbyl palmitate, and the first two indexes were higher than that of TBHQ. Thus, L-ascorbyl decanoate was demonstrated to be a potential food antioxidant.