A three-dimensional analysis aimed at enhancing the thermal performance of a double-layered microchannel heat sink by using a nanofluid and varying the geometric parameters has been conducted. A system of fully elliptic equations that govern the flow and thermal fields are solved using the finite volume method. The analysis indicates that the dominant factors determining the thermal resistance of the channel include the type of nanofluid; particle volume fraction; geometric parameters of the channel. such as the channel number, channel width ratio, channel aspect ratio; and pumping power. The results indicate that the greatest enhancement in channel cooling can be expected when an Al2O3-water nanofluid is used. The thermal resistance of the channel can be minimized by properly adjusting the particle volume fraction under various pumping powers; the minimum thermal resistance depends on the geometric parameters. The study also reveals that the relationship between the thermal resistance and channel number, channel width ratio, or channel aspect ratio exhibits a decrease followed by an increase. The thermal performance of the channel can usually be improved by decreasing the channel number or channel aspect ratio, or increasing the channel width ratio. Finally, increasing the pumping power reduces the overall thermal resistance. An Al2O3 (1%)-water nanofluid shows an average improvement in thermal performance of 26% over that of pure water for a given pumping power. However, the design's effectiveness declines significantly under high pumping power. In particular, the thermal resistance obtained by employing nanofluids was not necessarily lower than that of water under all pumping powers, but it can be reduced by properly adjusting the geometric parameters under optimal conditions. (C) 2012 Elsevier Ltd. All rights reserved.