Sterilization is widely used for eliminating all of microbes in food industry because of the advantages including high efficiency and convenience of use. In this study, we evaluated the antioxidant effects of two ethanol extracts prepared from the original and the sterilized Saccharina japonica (S. japonica, SJE and SSJE). In result, SSJE showed the improved extraction yields and protein and phenol contents, compared to those of SJE. Also, SSJE exhibited the higher radical scavenging activities against 2,2- azino-bis (3-ethylbenzthiazoline)-6-sulfonic acid (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) with IC50 values of 0.16± 0.00 mg/mL and 0.23±0.00 mg/mL than those of SJE. In addition, SSJE improved the reducing power and the oxygen radical absorbance capacity (ORAC) value, compared to SJE. Moreover, SSJE more strongly suppressed H2O2-induced intercellular reactive oxygen species (ROS) production compared to SJE without cytotoxicity in Vero cells. Therefore, our results suggest that the application of sterilization process to S. japonica leads to the improvement of antioxidant effects and SSJE may be used as a potential source of natural beneficial functional foods.