Abstract: Swelling deformation tests of a commercial bentonite are performed in distilled water and NaCl solution with different concentrations. The results show the swelling characteristics decrease with the increase of solution concentration. Study on the results finds that the greater concentration of inundation solution exerts greater osmotic suction . Increase in osmotic suction apparently acts as an equivalent effective stress that favors a reduction in swell potential of the compacted bentonite specimens. It means that, the effective stress includes the part of applied loads p and the part of osmotic suction p. The relationship between montmorillonite void ratio em and effective stress pe can be represented by a unified curve for bentonite in salt solution with different concentration and distilled water, and the part of osmotic suction p=(p/)0.66.