A numerical investigation is carried out to understand the effect of aspect ratio, as well as assisted buoyancy on flow and heat transfer characteristics in a vertical duct with rectangular cross section. Results are obtained for mixed convection flow with imposed inlet flow rate in a symmetrically heated duct with uniform wall temperature. The flow and thermal patterns are presented and discussed for various aspect ratios and buoyancy parameters. The critical value of buoyancy parameter (Gr/Re), above which the flow reversal starts to occur, decreases with increasing aspect ratio (L/W) for 0 < L/W < 0.75, and increases with increasing aspect ratio for 0.75 < L/W < 1. The position of the stagnation point and the profile of the recirculation cells for the flow reversal are significantly affected by the aspect ratio of the rectangular cross section. The influence of aspect ratio and buoyancy force on the local and average Nusselt number of the walls is also investigated. (C) 2014 Elsevier Ltd. All rights reserved.