The unsaturated soil is a heterogeneous and non-uniform material which is composed of solid-phase, liquid-phase and gas-phase, i. e., a multi-phase material. In this paper the relationship among inter-particle force, suction and Bishop's parameter χ in effective stress equation are derived based on Kitamura Model which is based on the mechanical & probabilistic consideration on the scale of particle size. Then the numerical experiments are carried out to obtain these relations and the discussion on the obtained results is done. It is found that the contribution of suction to inter-particle force at contact point depends on the grain-size distribution and void ratio of unsaturated soil.
