Laminar liquid-water flow and heat transfer in three-dimensional wavy microchannels with rectangular cross section are studied by numerical simulation. The flow field is investigated and the dynamical system technique (Poincare section) is employed to analyze the fluid mixing. The results show that when liquid coolant flows through the wavy microchannels, secondary flow (Dean vortices) can be generated. It is found that the quantity and the location of the vortices may change along the flow direction, leading to chaotic advection, which can greatly enhance the convective fluid mixing, and thus the heat transfer performance of the present wavy microchannels is much better than that of straight microchannels with the same cross section. At the same time, the pressure drop penalty of the present wavy microchannels can be much smaller than the heat transfer enhancement. Furthermore, the relative wavy amplitude of the microchannels along the flow direction may be varied for various practical purposes, without compromising the compactness and efficiency of the wavy microchannels. The relative waviness can be increased along the flow direction, which results in higher heat transfer performance and renders the temperature of the devices much more uniform. The relative waviness can also be designed to be higher at high heat flux regions for hot spot mitigation purposes. (C) 2010 Elsevier Ltd. All rights reserved.