This paper focused on the hydraulic and thermal characteristics of fractal tree-like microchannels with different ARs (aspect ratios) for the Reynolds numbers ranging from 150 to 1200. And the fractal microchannels with two branches and straight microchannels with the same heat transfer area were also compared in terms of the pressure loss, mean heat transfer coefficient, and COP (coefficient of performance). The experimental results showed that the fractal tree-like microchannels had a much higher heat transfer coefficient than that of straight microchannels at the cost of a much higher pump power. And for fractal and straight microchannels, the AR of microchannels was also found to have a relatively large impact on both the pressure loss and heat transfer. (C) 2012 Elsevier Ltd. All rights reserved.