Preferential adsorption of nanorods onto smooth walls is investigated using dissipative particle dynamics in the absence of specific attraction and a depletant. Although the translational and rotational entropy of nanorods is significantly reduced after adsorption, the effective attraction between the nanorod and wall is clearly identified based on the distribution profile of rods. As the rod length increases, the attractive interaction grows stronger and clusters of aligned nanorods can emerge on the smooth wall. However, the presence of a depletion zone of nanorods adjacent to the adsorbed layer gives zero surface excess. These two regions correspond to the primary minimum and maximum mean force potentials observed. Since adsorbed nanorods lose their rotational and translational entropy, the strong adsorption of long nanorods has to be attributed to the entropy gain associated with the increase in free volume for the solvent in this athermal system. Nonetheless, as the surface roughness is present, entropy-driven attraction is lessened, similar to the depletion force between colloids.