The flow characteristics of water in filleted microchannels were simulated based on CFD method. The flow pressure drop at different aspect ratioand Re number were rearranged on the simulating results with laminar flow model. The results indicated that the pressure drop enlarges with the increase of in the case of the constant width of the microchannel. Within the range of Re number of interest, Poiseuille number of the flow is constant for different Re, but decreases with increasing aspect ratio. An equation was fitted to describe the relationship between Po number and aspect ratio, i.e. .