We examined ethanol extracts prepared from the Korean marine algae belonging to the Sargassaceae family for their inhibitory effects on ${alpha}$-glucosidase activity and free radicals in vitro. Among five marine algae, the extracts of Sargassum yezoense were found to possess strongly ${alpha}$-glucosidase inhibition and free radicals scavenging activities. Two compounds were isolated via bioactivity guided isolation and tested for their effects on ${alpha}$-glucosidase, DPPH, $ABTS^{+}$ and $Photochem^{(R)}$ analysis. Their chemical structures were elucidated by spectral analysis and direct comparison with authentic compounds; their structures were identified as sargaquinoic acid (1) and sargahydroquinoic acid (2). The inhibitory effects of compound 1 and 2 ($IC_{50}$ value:14.2 and 12.8 ${mu}M$, respectively) on ${alpha}$-glucosidase were more potent that of deoxynojirimycin as a positive control ($IC_{50}$ value:18.0 ${mu}M$). All compounds displayed antioxidative activity which was measured by DPPH, $ABTS^{+}$ and $Photochem^{(R)}$ apparatus.