Direct numerical simulations are conducted for the flow which passes between two opposite plates at the Reynolds number of 2800 and Prandtl number of 0.71 (air). The six different computational domains considered in the present study are made up of one General dimple case and Riblet-mounted dimples cases with five different riblet angles. Among six different cases, the most superior thermo-aerodynamic performance in terms of the volume goodness factor is obtained for the case of Riblet-mounted dimple with the riblet angle of 60 degrees regarding the improvement of compactness of a heat exchanger matrix. For the case with the riblet angle of 60 degrees, the pressure loss in terms of the Fanning friction factor is substantially reduced, and the heat transfer capacity in terms of the Colburn j factor has almost the same level as that of the General dimple case. The Riblet-mounted dimple is a more competitive surface shape under the carefully designed riblet angle regarding the improvement of compactness of a heat exchanger matrix. (C) 2010 Elsevier Ltd. All rights reserved.