Abstract: The objective of this study is to investigate the effect of a nonionic surfactant,Triton X-100(0.1%),on the capil lary pressure-saturation relationships in two-phase system.This paper sets out the relationships between the capillary pressure and the saturation of water-air and water-oil system with surfactant Triton X-100.Experimental results show that,compared with the system without surfactant,the values of capillary pressure of the system with surfactant are lower under the same saturation,which indicates that lower capillary pressure is enough to drive the same volume of wetting-phase liquid.The data fitting based on the van Genuchten model shows that,when the relationship between capillary pressure and saturation without surfactant is known,the fitting values with a scaling factor involving interfacial tension accord with the measured values for water-air system with surfactant; however,for the water-oil system with surfactant,the scaling approach making fitting values accord with the measured values dependeds on the porous media.