The present invention relates to the obesity-curative or obesity-preventive effect of a Polygonum cuspidatum butanol fraction and a Polygonum cuspidatum ethylacetate fraction, and more particularly, to a pharmaceutical composition and a functional food for treating obesity, the pharmaceutical composition and the functional food comprising a Polygonum cuspidatum butanol fraction (POCU-1b) and an ethylacetate fraction as active ingredients, wherein the Polygonum cuspidatum butanol fraction and the ethylacetate fraction inhibit effectively the activity of pancreatic lipase, an important enzyme involving in fat absorption in a living body, and have excellent inhibitory effect on fat absorption in the short term fat absorption-inhibitory animal experiments using lipid emulsions.