Mechanical behaviors of bentonite that is widely used in geotechnical engineering, are important for engineering safety. The deformation and strength characteristics of bentonite under unsaturated state or in a salty environment are influenced by osmotic suction and matric suction. A unified quantitative expression of effective stress including the osmotic suction and matric suction terms is required. In this paper, structural fractal models for bentonite were developed and the effective stresses of saturated bentonite in salt solution and unsaturated bentonite in deionized water are derived, respectively. Using the proposed effective stresses, the swelling deformation and the peak shear strength of bentonite can be depicted by a unified expression. Further, the unified effective stress for unsaturated soil in salt solution was deduced in conjunction with existing studies. The results of this study are important for quantifying the mechanical behaviors of bentonite under a complex state.