Abstract: A numerical research was conducted to analyze the friction constants of laminar flow of water in smooth trapezoidal silicon microchannels with different aspect ratios. Based on the Navier Stokes equation, the friction factors of 25 different trapezoids were calculated using SIMPLER algorithm and domain extension method. Compared the calculated results with the experimental results of H.Y. Wu and P. Cheng, it is shown that the calculated results are in reasonable agreement with the experimental results given by H.Y. Wu and P. Cheng, and the friction constants of these microchannels increase with the aspect ratios increasing. Therefore, it is confirmed that the Navier-Stokes equations are still valid for the laminar flow of water in smooth silicon microchannels having hydraulic diameter as small as 25.9 μm.