Abstract: Computational fluid dynamics simulation was conducted in studying characteristics of fluid flow and heat transfer of microchannel cooler installed in a solid-state laser. Two-dimensional and three-dimensional physical models were developed, respectively, based on the cooler structure. Fluid flow within the microchannel was first investigated with the two models, then effects of Reynolds number and heat generation rate of crystal slice on fluid flow and heat transfer in the cooler were numerically examined. Results showed that the two-dimensional model was sufficient to describe the fluid flow and heat transfer behaviors in the laminar flow region in the rectangular microchannel between two parallel planes, and the three-dimensional model was more suitable for the transition region. When the Reynolds number was increased to the transition point, the effect of heat transfer from fluid flow was significantly enhanced. With the Reynolds number rising, the effect of the heat on minimum pressure required within the channel was gradually reduced. The change in the heat had an important influence on fluid flow, but little influence on Nusselt number and total pressure drop of the channel.