The potential antidiabetic activity of ethyl acetate extract of the leaves of Lagerstroemia speciosa (LSL) wasinvestigated by ααααα-amylase and ααααα-glucosidase inhibition assay. Six pentacyclic triterpenes (oleanolic acid, arjunolicacid, asiatic acid, maslinic acid, corosolic acid and 23-hydroxyursolic acid) were isolated from LSL. Theirstructures were determined by spectroscopic analysis and their ααααα-glycosidase and ααααα-amylase inhibitory activi-ties were investigated. They exhibited no or weak inhibitory activity against ααααα-amylase and middle ααααα-glucosidaseinhibitory activities. Corosolic acid, which shows best bioactivity against ααααα-glucosidase (IC50 ===== 3.53μμμμμg/mL),contributes most to the ααααα-glucosidase inhibitory activity of EtOAc extract. The kinetics of inhibition ofcorosolic acid was also discussed. Results from this study might provide the scientific evidence for LSL for thetreatment of diabetes in traditional medicine