Abstract: Unsaturated soil is a three-phase porous media. The interfaces between every two phases, especially between liquid and gas phase, have important influence on soil behavior. The interfacial form and effect of soil are discussed and the expressions for interfacial work and specific interfacial area are presented. By introducing the interfacial work into the existing work input equation, the Helmholtz free energy of unsaturated soils with interfacial effect can be derived. Based on the free energy equation, the stress variables considering interfacial effect for solid and liquid can be obtained. The influences of interfacial area on flow equation of liquid phase are discussed, and it is pointed out that the gas-liquid interface area should be introduced as a third variable in a complete soil water retention curve (SWRC). The traditional SWRC is a projection of the three-dimensional expression in the (s, S_r) plane. The effective stress considering specific area can be calculated by the relationship between specific area and SWRC. The proposed equation of effective stress is used to simulate experimental data in the literature, and results show our findings are in good accordance with practice. Different from the existing phenomenology studies, the deductive process has a rigorous theoretical basis in this paper. The results shown that the complete expression of effective should consider the effects of internal force in soil, which including not only matric suction, but also the other force related to the specific interfacial area.