he petroleum carboxylic acid in vacuum residue distillates of crude oil was separated by the extraction with column chromatogram and anion exchange resin.Composition and structure of naphthenic acids were obtained by the use of IR techniques. Petroleum acids with iso-butane reagent gas was introduced to ion source that can be ionized by chemical ionization（CI）-mass spectrometer（MS）. These ions can be used to calculate the molecular mass for each acid as previously described. Based on mass spectrum of pure fatty and naphthenic acid and combined with z-series formula C _nH _2n+zO ₂.The type of naphthenic acids can be classified into fatty, mono- ,bi- …hexa-cyclic according to their mass spectrum and data. The results indicate that naphthenic acid is the main composition of vacuum residue and the molecular weight distribution of petroleum acids in this distillate is about 198~540,and the carbon number distribution is about C ₁₂~C ₃₇.The contents of tri-cyclic and tetra-cyclic naphthenic acids are higher than others.