Various microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be suitable to enhance the heat transfer, and are successfully applied in heat-exchange devices. In this article, three kinds of water-cooled microchannel heat sinks, such as a rectangular straight microchannel heat sink, a single-layer wavy microchannel heat sink and a double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude on heat transfer, pressure drop, and thermal resistance are also observed. Results show that for removing an identical heat load, the overall thermal resistance of the single-layer wavy microchannel heat sink decreases with increasing volumetric flow rate, but the pressure drop is increased greatly. At the same flow rate, the double-layer wavy microchannel heat sinks can reduce not only the pressure drop but also the overall thermal resistance compared to the single-layer wavy microchannel heat sinks. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu₀ )/(f/f ₀ ), and (Nu/Nu₀ )/(f/f ₀ ) ^1/3 , are applied and some controversial results are obtained.