The objective of this paper is to report a numerical investigation into the heat transfer of a buoyancy aided flow in a rod-bundle-like channel. The flow field is simulated using large eddy simulation (LES) with WALE SGS model and the buoyant force is taken into account using the Boussinesq approximation. The general trend of the effect of buoyancy on the overall heat transfer is similar to that in a pipe flow, but the effect on the regional heat transfer varies greatly. This has resulted from a number of interplaying factors, including, the redistribution of the mass flow in the various sub-channels, the non-uniform buoyancy effects on turbulence in different regions of the domain and the behaviour of the large flow structures in the flow channel. These factors together make the effect of buoyancy on heat transfer in the considered flow channel really complicated, while the last factor has been found to have the most pronounced effect in most cases studied. (C) 2017 The Authors. Published by Elsevier Ltd.