Pressure drop and heat transfer characteristics, in the form of fㆍRe and Nu respectively, of hydrodynami-cally developed laminar flow through a square channel containing a bundle of 25 tubes with a uniform surface tempera-ture are studied by solving the momentum and energy balance equations in finite difference forms, using a forward marching implicit finite difference method. In this work, the influences of the tube pitch on the friction factor and the Nusselt number of the 5×5 tube bundles with a constant tube diameter are investigated. At a fixed tube diameter the value of fㆍRe increases with increasing the tube pitch and then decreases as the tubes approach the shell well. Leveque type asymptotic solutions for predicting the heat transfer coefficient for thermal entrance region are valid only in the range of very large Graetz number, if the tube pitch is very small. In this case the slope of the curves in log Nu vs. log Gz diagram is partly greater than 1/3. The impact of the tube pitch on the heat transfer rate is great in case of the small tube pitch. The Nusselt number of the smaller tube pitch for the fully developed flow is about 10 times as small as that for the larger tube pitch.