Alpha-glucosidase inhibitors are oral antihyperglycemic agents that act on alpha-glucosidase competitively, delaying intestinal carbohydrate absorption and lessening postprandial increases in glucose levels. In the search for new classes of $alpha$-glucosidase inhibitors extracted from natural resources, 420 different plant extracts were evaluated for their inhibitory effect on $alpha$-glucosidase, in vitro. Amongst the tested extracts, the seed extract of Paeonia lactiflora (Paeoniaceae) demonstrated a significant degree of inhibition on the enzyme, according to dose. Extensive bioassay-guided fractionation of the extract resulted in the isolation of six materials, luteolin (1), resveratrol (2), trans-$varepsilon$-viniferin (3), gnetin H (4), suffruticosol A (5) and suffruticosol B (6), that are active agents for $alpha$-glucosidase inhibition. Resveratrol (2) is a well-known naturally occurring hydroxystilbene, particularly abundant in wine and some related stilbene oligomers. Materials (3-6) exhibited relatively strong inhibition of $alpha$-glucosidase and their $IC_{50}$ values were calculated as: 0.92mM (2), 3.15mM (3), 1.15 mM (4), 2.53mM (5) and 3.15mM (6). In contrast, the ($IC_{50}$) value of the reference drug, acarbose, was estimated as 8.28mM in vitro.