Abstract: Oil film life, rate of oil film thinning, water-oil interfacial properties and their effects on demulsification of model petroleum emulsions(O/W) in the presence of demulsifiers are studied. The results indicate that the interfacial elasticity decreased with the addition of demulsifiers, resulting a decrease of both the strength and the thickness of the oil film, and hence leading to a shorter film life. The film will break down when the film thickness reaches to a critical value by film thinning .For the demulsifiers in the same series, the interfacial elasticity of the film lowered with increasing demulsifier concentration initially, and then kept constant after the concentration reached a definite value. For the demulsifiers in different series, the more the interfacial elasticity lowered, the better the dewatering effect will be. The dewatering rate is correlated with the interfacial elasticity. An action mechanism of the demulsifiers was proposed according to the experimental results.