An extract of Polygonum multiflorum Thunb. ex Murray var. hypoleucum (Ohwi) or of Polygonum hypoleucum (Ohwi) for improving metabolic syndrome is prepared by the following method: (a) providing fresh or dry Polygonum multiflorum Thunb. ex Murray var. hypoleucum (Ohwi) or Polygonum hypoleucum (Ohwi) in partial or whole plant ; (b) extracting the partial or whole plant by a solvent to obtain a crude extract; (c) condensing and drying the crude extract to obtain an extract product; and (d) collecting the extract product. The present invention also relates to a pharmaceutical and a food composition having the above extract for improving the metabolic syndromes. The extract of the present invention can be easily prepared and is demonstrated to modulate the levels of both blood glucose and blood lipid, to inhibit the activities of glycolytic enzymes and acetyl-CoA carboxylase, and to stimulate acyl-CoA oxidase. The extract of the present invention is capable of improving metabolic syndromes.