The paper presents the geometric optimization of the micro-heat sink with straight circular microchannels with inner diameter of D-i = 900 mu m. The inlet cross-section has a rectangular shape and positioned tangentially to the tube axis with the four different geometries. The fluid flow regime is laminar and water with variable fluid properties is used as a working fluid. The heat flux spread through the bottom sink surface is q = 100 W/cm(2). Thermal and hydrodynamic performances of the heat sink are compared with results obtained for conventional channel configuration with lateral inlet/outlet cross-section. Besides, the results are compared with the tangential micro-heat sink with D-i = 300 mu m. For all the cases, the thermal and hydrodynamic results are compared on a fixed pumping power basis. (C) 2010 Elsevier Ltd. All rights reserved.