Saccharina japonica (SJ) has been used as an oriental medicine in Korea due to its beneficial biological roles. In this study, we evaluated the antioxidant effects of Lactobacillus plantarum-fermented SJ (FSJ) by ABTS+ radical scavenging, oxygen radical absorbance capacity (ORAC), hydrogen peroxide, and electron spin resonance (ESR) assays. First, FSJ was enzymatically hydrolyzed by a carbohydrase (Celluclast) and a proteinase (Trypsin) and both enzymatic extracts (CFSJ and TFSJ) showed the improved extraction yields, compared to the water extract (WFSJ). TFSJ also showed the higher DPPH and hydroxyl radical scavenging activities than those of CFSJ as well as their alkyl radical scavenging activities. In addition, CFSJ and TFSJ showed the 0.30±0.02% and 0.26±0.03% of ORAC values and the higher ABTS+ radical scavenging activities in a dose-dependent manner. According to these results, we suggest that FSJ can be used as a beneficial material to improve oxidation in various food or oriental medicine industries.