Abstract: Demulsifiers, alkyl phenol-formaldehyde polyoxyethlene polyoxypropylene ethers were synthesized for desalting of Xingjiang crude oils. The effect of molecular structure of the demulsifiers on its desalting ability as well as on the interfacial tension of crude oil-water interface was discussed. The results showed that the desalting efficiency on crude oils is largely affected by the molecular weight and the hydrophilic-lipophilic-balance (HLB) value of the demulsifiers. The best desalting efficiency was obtained at certain value of HLB. It was also shown that for one certain crude the best HLB value of two series phenolic-formaldehyde demulsifiers are close. The interfacial tension has opposite changing tendency with the desalting efficiency. An adsorbing state model of this kind of demulsifiers at crude oil-water interface is suggested to explain the phenomenon above.