The heat transfer and flow characteristics of the microchannel heat sink (MCHS) with bidirectional ribs (BRs) are experimentally and numerically studied in the present paper. The BR, composed of vertical rib (VR) and spanwise rib (SR), can interrupt the thermal boundary layer and induce recirculation in both vertical and spanwise directions. Its cooling effectiveness is compared with that of the widely-used VR and SR for the Reynolds number ranged from 100 to 1000. The results show that the Nussalt number of the microchannel with BRs (BR-MC) is up to 1.4-2 and 1.2-1.42 times those of microchannels with VRs (VR-MC) and SRs (SR-MC), respectively. This implies that the BR can strengthen the heat transfer more sufficiently. Meanwhile, the utilizing of BR gives rise to the larger pressure drop penalty due to its broader obstruction areas. In addition, the higher relative rib height of VR (e(VR)) and relative rib width of SR (e(SR)) are revealed to enhance the heat transfer but induce pressure drop in the BR-MC. The thermal enhancement factor can keep larger than 1 when e(VR) < 0.316 and 0.026 < e(SR) < 0.357. (C) 2019 Elsevier Ltd. All rights reserved.