Abstract: Four kinds of asphaltenes were obtained from four different heavy oils with n-pentane solvent. Their functional structures were characterized by FTIR spectroscopy, and the basic structural parameters of the asphaltenes were determined by 1H-NMR, then the average molecular formula of the asphaltene were given by molecular weight and element content. The dipole moments were also measured to indicate the polarities of the asphaltene molecules. The results show that the presence of heteroatom in asphaltene makes the asphalt form the polar groups with high polarity, and the higher the content of impurity atoms, the greater the polarity, the stronger the association, the higher the association numbers. In addition, molecular diameters were measured by two methods: intrinsic viscosity method, and relative density and relative molecular mass method. With the increase of relative molecular mass, the molecular size of asphaltene becomes larger. However, the molecular diameter measured by viscosity method is related to the selected solvents and usually is larger than the size obtained by relative density and relative molecular mass method, and the latter method can well reflect the molecular size.