Some Sri Lanka plant stuffs were examined regarding in vitro and in vivo alpha-glucosidase (AGH) inhibitory actions. According to the results, water extracts and methanol extracts of dried fruits of Nelli (Phylanthus embelica), methanol extracts of dried flowers of Ranawara (Cassia auriculata), and water extracts of latex of Gammalu (Pterocarpus marsupium) were found to have a potential AGH inhibitory activity. In particular, Ranawara methanol extract showed the strongest AGH inhibitory activity in vitro preferably on maltase giving an IC(50) value of 0.023 mg/mL and inhibited the maltase activity competitively. As a result of single oral administration of Ranawara (C. auriculata) methanol extract in Sprague-Dawley rats, a significant and potent lowering of blood glycemic response toward maltose ingestion was observed at 30 min after dosing of 5 mg/kg, thus, concurrently suppressed insulin activity. The ED(50) of the extract (4.9 mg/kg) clearly indicated that the antihyperglycemic effect was as potent as that of therapeutic drug, acarbose (ED(50) 3.1 mg/kg).