Three-dimensional flow structure in a rectangular channel with a built-in oval tube is quite complex due to the formation of horseshoe vortices near the front stagnation region of the tube and the Karman vortices behind it. The presence of a winglet pair in conjunction with the oval tube makes the flow structure even more complex. The interaction of the horseshoe vortices with the longitudinal vortices generated by the winglets and the interaction of the longitudinal vortices with the transverse Karman vortices culminate as a flow field dominated by the tangle of vortices. The existence of more than one pair of longitudinal vortices would further complicate the flow structure and the heat transfer behavior in the channel. The interaction between the secondary flows generated by the tube and by the winglets, as well as the interaction of the longitudinal vortices due to different arrangement of the winglets, play an important role in heat transfer enhancement. In the present article, effect of the streamwise as well as cross-stream location of the winglet pairs on heat transfer is investigated. For two pairs of winglets, possible configurations have been envisaged. All the possible arrangements have been investigated, and the optimal configuration has emerged. © 2003, Taylor & Francis Group, LLC. All rights reserved.