Antioxidative activities of Chlorophyta and Phaeophyta in Jeju Island were measured by superoxide anion ($O_2^{{cdot}-}$), hydroxyl radical ($HO^{cdot}$), hydrogen peroxide ($H_2O_2$) and DPPH free radical scavenging assays. Methanolic and aqueous extracts of the seaweeds were prepared at both temperatures, higher (70$^{circ}C$) and room temperature (20$^{circ}C$), and screened for the construction of an extract bank from seaweeds in Jeju Island. A variety of extracts showed positive effect against reactive oxygen species (ROS). Especially, Sargassum thunbergii methanolic extract at 70$^{circ}C$ (70ME, 97.41%), S. fulvellum methanolic extract at 20$^{circ}C$ (20ME, 84.66%), Codium fragile aqueous extract at 70$^{circ}C$ (70AE, 96.61%) and S. thunbergii 20ME (97.44%) exhibited the highest scavenging activities against $O_2^{{cdot}-}$, $HO^{cdot}$, $H_2O_2$ and DPPH free radicals, respectively. Total phenolic contents also examined but did not show a positive correlation with ROS scavenging abilities (except for a few extracts). These results indicate that further investigation is needed to identify and purify the responsible antioxidative components.