The effects of fin spacing on four-row annular-finned tube bundles in staggered and in-line arrangements are investigated by the three-dimensional numerical study. Renormalization group theory (RNG) based k-epsilon turbulence model is allowed to predict the unsteady flow and conjugate heat transfer. According to the flow visualization results, the boundary layer developments and horseshoe vortices between the fins are found to be substantially dependent on the fin spacing to height ratio and Reynolds number. The heat transfer and pressure drop results for various fin spacings are presented. In addition, the numerical results are compared with the existing correlations. (C) 2003 Elsevier Ltd. All rights reserved.