Abstract: For obtaining the basic characteristic of flow and heat transfer in the rectangular microchannels,experiment was executed under different hydraulic diameters,and a great deal of numeration simulations were carried out and validated experimentally.The transformation laws of Nusselt number and resistant coeffcient were obtained.The critical Reynolds number was brought forward to about 1 200～1 300 in the studied scale range.Meanwhile,the correlation of Nusselt number and resistant coeffcient within the studied scale range were built by multivariate linear regression.The optimization procedure(genetic algorithms) was carried out to optimize the structure of microchannels,providing beneficial theoretical and experiment data for engineering application.