Abstract: This paper starts with a discussion of the effective stress functions for saturated soils, and then some situations of the theoretical predicting uncertainties in soil mechanics are stated. The relationship between the total stress and stress of each phase in unsaturated soils is developed based on three-phrase equilibrium equations of a representative element volume (REV). The expression of average soil skeleton stress, defined as effective stress, for unsaturated soils is then derived. This expression is consistent with the effective stress equation in unsaturated soils derived based on deformation work in the energy conservation equation. Then some discussions on the effective stress of unsaturated soil are given.