Understanding colloid transport behavior in unsaturated porous media is important because mobile colloid-contaminant complexes and colloidal pathogens can degrade groundwater quality. Visual evidence suggests that colloids are retained at the air-water meniscus-solid (AWmS) interface in unsaturated porous media, but there is no quantitative theoretical explanation of this retention mechanism to date. A theoretical study is presented here to quantify energy potentials represented by capillary forces on colloids at the AWmS interface in unsaturated porous media. Our calculations indicate that the capillary energy potentials for colloids retained in films or at the AWmS interface range from 107 to 108 kt, which are several orders greater than the Derjaguin-Landau-Verwey-Overbeek energy potentials. Capillary forces for colloids at the AWmS interface can be decomposed into a force that pushes the colloid back in the bulk solution and one that pins the colloid against the surface of the grain. A friction force generated between the colloid and grain can prevent the colloids from moving back into solution. When drag forces can be ignored, colloids will remain at the AWmS interface as long as the static friction coefficient is greater than the tangent of the water-grain contact angle. It is independent of the surface tension.