A unique reentrant microchannel heat sink is developed in this study. It consisted of 14 parallel Omega-shaped reentrant copper microchannels with a hydraulic diameter of 781 mu m. Single-phase convective flow and heat transfer performance of reentrant microchannels (REEM) were comprehensively explored both experimentally and numerically, and their cooling effectiveness was compared with conventional rectangular microchannels. Utilizing deionized water as the coolant, tests were conducted at Reynolds number of 150-1100, three different heat fluxes, and two inlet temperature of 33 and 60 degrees C. The results show that the averaged Nusselt number of reentrant microchannels increased up to 39% and the total thermal resistance decreased up to 22% as compared to the rectangular counterpart. Moreover, the reentrant microchannels also maintained notably lower wall temperatures, while they just incurred slightly larger or comparable pressure drop penalty. The above heat transfer enhancement is associated with the flow separation caused by the throttling effects, the acceleration of fluid in the main flow and the intensification of fluid mixing in the unique reentrant configurations of REEM. This study sheds some lights on the design of advanced microchannel heat sinks and is believed to be of practical importance. (C) 2015 Elsevier Ltd. All rights reserved.