A Myagropsis myagroides (Mm) methanol extract showed α-amylase inhibitory activity of 13% at a concentration of 5 mg/ml. Results showed that the hexane fraction from the Mm methanol extract exhibited α-amylase inhibitory activity with an IC50 value of 4.24 mg/ml. The hexane fraction was separated using silica-gel column chromatography, and six subfractions were obtained. The fraction eluted with CHCl3:MeOH = 50:1 showed the highest α-amylase inhibitory activity with an IC50 value of 0.72 mg/ml. This fraction was purified using Sephadex LH-20 column chromatography and an octadecyl silica (ODS) Sepak cartridge, obtaining seven subfractions. Fraction (Fr.) 4 also showed a strong α-amylase inhibitory activity with an IC50 value of 0.75 mg/ml. Fr. 4 was purified by Sephadex LH-20 column chromatography and ODS Sepak cartridge, obtaining six subfractions. Fr. 4-2 was identified as sargachromanol I with an IC50 value of 0.40 mg/ml, and the inhibition pattern analyzed from Lineweaver-Burk plots revealed it to be an uncompetitive inhibitor. These results suggest that Mm has potential as a natural antidiabetes agent.