This research investigated the impact of particle size on colloid retention in water unsaturated porous media using column experiments. Three different-sized silica sols were used as the model colloids in this research. Colloid column transport experiments were conducted under steady-state water flow conditions with effective water saturation ranging from 0.2 to 1.0 in two different-sized porous media. Colloid transport was described by the mobile-immobile two-region transport model. Under saturated conditions, minimal colloids were retained in the porous media. Under water unsaturated conditions, the observations were closely related to system water saturation. At high water saturation (i.e., effective water saturation of 0.80), colloid retention decreased with the increase of colloid size. At intermediate water saturation (i.e., effective water saturation of 0.50), the effect of particle size on colloid retention was not obvious. At low water saturation (i.e., effective water saturation of 0.20), colloid retention in the porous media increased with the increase of particle size. Under constant chemical conditions, colloid retention in the unsaturated porous media was believed to be determined by a balanced effect of shear forces and physical constraints, which was a function of both particle size and water saturation.