Olive (Olea europaea L.) is recognized as a folk medicine for diabetes in Europe. The inhibitory action of an ethanol extract of olive leaves (OEE) on the activities of human amylases was examined in vitro. OEE inhibited the activities of α-amylases from human saliva and pancreas with IC₅₀ values of 4.0 and 0.02 mg/ml, respectively. Two anti-α-amylase components were purified from a 50% ethanol soluble fraction of OEE using Sephadex LH-20 column chromatography. One was identified as luteolin-7-O-β glucoside and the other as luteolin-4′-O-β glucoside. The 50% ethanol insoluble fraction of OEE was dissolved in 98% ethanol and fractionated using Cosmosil C18-OPN column chromatography. The anti-α-amylase component purified by this chromatography was identified as oleanolic acid. Both luteolin and oleanolic acid have an inhibitory effect on postprandial blood glucose increase in diabetic rats. Olive leaves suppressed the elevation of blood glucose after oral administration of starch in borderline volunteers (fasting blood glucose: 110–140 mg/dl), and thus they may be a useful food supplement for the prevention of diabetes.