Abstract: The initial boiling point (IBP) to 400 ℃ fractions of Beijiang highly acidic crude oil were separated into 8 fractions. Their total acid numbers (TAN), the aromatic structures, and the change of interfacial tension of oil-water phases for the fractions in different conditions were investigated. The oil-water interfacial tensions before and after the deacidification were compared. The results showed that the higher the TAN and content of aromatic structure were, the lower the oil-water interfacial tension was, and pH has different effects on light and heavy fractions. After acid removal, the TAN of the fractions was decreased evidently and more than 60% of the naphthenic acid could be removed. The oilwater interfacial tension increased dramatically.