Heat transfer and fluid flow over in-line bundles of cylinders have been numerically simulated. Flow is assumed as incompressible, two-dimensional and laminar. Analysis has been carried out for Reynolds numbers of 50, 100, 150 and pitch to diameter ratios (PDR) of 1.5 and 2.0 with a Prandtl number of 0.71. The effect of Richardson number (Ri = Gr/Re-2) on the flow and heat transfer has been investigated for Ri = -1.0, -0.5, 0.0, +0.5 and +1.0. Streamlines, temperature contours, average Nusselt numbers, average friction coefficients and pressure distribution around the cylinders are presented. It is observed that there is a considerable effect of buoyancy over tube bundles.