Microbial conversion of unsaturated fatty acids often leads to special changes in their product structures and in biological potential. In our continuous effort to screen natural products for their antimicrobial and enzyme inhibitor activities, we found that 10-hydroxy-8(E)-octadecenoic acid (HOD) exhibited strong anti-α-glucosidase (EC 220.127.116.11) activity. HOD is an intermediate in the bioconversion of oleic acid to 7,10-dihydroxy-8(E)-octadecenoic acid (DOD) by a bacterial isolate, Pseudomonas aeruginosa (PR3). Diabetes mellitus is the most serious, chronic metabolic disorder characterized by defect in insulin secretion and action, which can lead to damage blood vessels and nerves. We analyzed the inhibitory activity of HOD and the commercially available antidiabetic remedy, acarbose. We found that HOD exhibited a better inhibition (IC50 0.07±0.12) on α-glucosidase as compared to acarbose (IC50 0.42±0.1). HOD showed competitive inhibition against yeast α-glucosidase. Our study is the first report on anti-α-glucosidase activity of HOD and could be helpful to develop medicinal preparation or functional food for diabetes and related symptoms.