This study was performed to investigate the inhibitory activity of Sargassum thunbergii (ST) against ${alpha}$-amylase and elucidate the availability of ST extract as a functional food agent. To test the inhibitory activity of ST against ${alpha}$-amylase, porcine pancreatic ${alpha}$-amylase and potato starch were used as substrates. It was revealed that ST crude ethanol extracts have high ${alpha}$-amylase inhibitory activity. Subsequently, ST crude ethanol extract was separated into five partition layers by solvent extraction: n-hexane, chloroform, ethyl acetate, butanol, and water. Chloroform and n-hexane fractions showed higher inhibitory activities than did acarbose (positive control). To confirm the changes in enzyme inhibitory activity by physical treatments, ST crude ethanol extract was subjected to heat, pH, and ${gamma}$-irradiation treatments. In all heat treatments with the exception of one ($121^{circ}C$, 15 min), the inhibitory activity was increased compared with the untreated group. With regard to pH stability, ST extract showed no significant changes at pH 4.6, but somewhat decreased inhibitory activity was revealed at pH 2, 8, and 10. On the other hand, ST ethanol extract was stable under ${gamma}$-irradiation under all conditions (3.20 kGy). In summary, ST ethanol extract can be used in the food industry as a natural ${alpha}$-amylase inhibitor.