Polygonum paleaceum Wall. ex Hook. f. is a small herb in Polygonaceae family. In southwest of China, the rhizomes of this plant were used as folk drug for the treatment of chronic gastritis, duodenal ulcers, dysentery, wound, pain, hemorrhage and irregular menstruation. The crude aqueous acetone extract exhibited high antioxidant activity (SC(50) = 16.72 micro/ml) in 1,1-diphenyl-2-picryldydrazyl (DPPH) radical-scavenging assay. With bioactivity-guided fractionations, 14 antioxidant phenolic compounds 1-14 were obtained from the air-dried rhizomes of this plant. Of them, compound 1, a new phenol named paleaceolactoside, was determined to be 3,5-dihydroxyl hexanoic acid 1,5-lactone 3-(6'-o-galloyl)-o-beta-D-glucopyranoside by detailed spectroscopic analysis. The radical-scavenging activity of all the isolated compounds was also described.