VOC (Volatile Organic Compound) gas extraction from the sub-soil is an effective skill to recover the soil contamination, but the detailed relationship between soil structure and mechanism of VOC sorption in the unsaturated zone is not solved. In this paper, the retardation factor R in the diffusion equation is formulated in consideration of the sorption balance of VOC between gas, liquid, and solid phases of soil model. Hereby, on the basis of the experiment of TCE (Trichloroethylene) adsorption for several model soils including one with aggregate structure, the movement of VOC in the soil could be represented by the application of basic soil parameters such as specific surface area, water content, and adsorption coefficients with gas-liquid, gas-soil, soil-liquid interfaces.