In this study, two- and three-dimensional numerical simulations were performed to investigate the effect of the flow structure in the wake of a square cylinder placed near a plane wall by applying a fully implicit finite-difference method to the Navier-Stokes equations. The gap ratio between the cylinder and the wall, G/D, was varied from 0.2 to 4 for the Reynolds numbers of 175,185 and 250. The role of the 3D structure on the lift and drag coefficients and Strouhal number was investigated. The results were compared with those of the 2D numerical simulations. The deviations of the 3D flow structure of the cylinder-wall pair from that of a single cylinder were also reported. At Re = 250, B type secondary vortices were determined in the wake region. At Re = 175 and 185, transition from A type vortex to fully periodic B type vortices was observed when the cylinder was brought closer to the wall. (C) 2009 Elsevier Ltd. All rights reserved.