Abstract: The petroleum carboxylic acids in distillates of Liaohe crude oil under 200~430 ℃ were separated by the extraction with alkali alcoholic solution and Freon 113 solvent .The relations to composition and structure of naphthenic acids were obtained by the use of IR and MS techniques. The result indicated that the molecular weight distribution of naphthenic acids in Liaohe distillates of 200~430 ℃ was about 142~692 Da, and the carbon number distribution was about C 8~C 48 . The contents of monocyclic ,bicyclic and tricyclic naphthenic acids were higher than others. A scientific and efficient method will be supplied for the further research in naphthenic acid corrosion and its mechanism related to geological chemistry.