This work uses an optimization procedure consisting of a simplified conjugate-gradient method and a three-dimensional fluid flow and heat transfer model to investigate the optimal geometric parameters of a double-layered microchannel heat sink (DL-MCHS). The overall thermal resistance R-T. is the objective function to be minimized, and the number of channels N, channel width ratio beta, lower channel aspect ratio alpha(t), and upper channel aspect ratio alpha(u) are the search variables. For a given bottom area (10 x 10 mm) and heat flux (100 W/cm(2)), the optimal (minimum) thermal resistance of the double-layered microchannel heat sink is about R-T = 0.12 degrees C/m(2)W. The corresponding optimal geometric parameters are N = 73, beta = 0.50, alpha(t) = 3.52, and, alpha(u) = 7.21 under a total pumping power of 0.1 W. These parameters reduce the overall thermal resistance by 52.8% compared to that yielded by an initial guess (N = 112, beta = 0.37, alpha(t) = 10.32, and alpha(u) = 10.93). Furthermore, the optimal thermal resistance decreases rapidly with the pumping power and then tends to approach an constant value. As the pumping power increases, the optimal values of N. ab and au increase, whereas the optimal beta value decreases. However, increasing the pumping power further is not always cost-effective for practical heat sink designs. (C) 2012 Elsevier Ltd. All rights reserved.