We present a new procedure to prepare high-quality colloidal crystals at a flat wall, which was oriented vertically in a colloidal suspension. The method is based on the depletion-induced effective attraction between the colloidal particles themselves and between the colloidal particles and a flat wall. The concentrations of the colloidal particles and the depletion agents (polymers in this case) were chosen in such a way that phase transition occurred not only at the wall but also in the bulk. In this way, good conditions were obtained to form high-quality colloidal crystals, consisting of several layers, at a flat wall in a relatively short time period. The crystals formed were characterized with atomic force microscopy.