Abstract: Owing to different effects of action, the internal forces of skeleton of saturated soils can be separated into two different sets in equilibrium. One is produced by pore water pressure, and the other by external loads. By taking each phase (i.e., soil skeleton and pore water) as an independent free body, the equilibrium differential equations can be derived by means of the equilibrium analysis for representative elementary volume. As a result, the skeleton stress equation, i.e., effective stress equation, is obtained. It shows that the essence of the effective stress equation is the force interaction between phases of the soil. The Skempton’s modified formula for effective stress equation, which is gotten from granular stress analysis, is inappropriate. The equivalent stress expressions which are derived according to the soil volume change or shear strength can also be formularized with skeleton stress and pore water pressure. The proposed method can be applied to unsaturated soils.