Investigated is the effect of slip on the friction number for water flow in microchannels with different D_h and aspect ratio, α. An analytical expression for the friction number in microchannels with a slip at the wall is derived from the solution of the Navier-Stokes equations. In addition, CFD numerical analyses are performed and the results are compared to those of the analytical solution and the reported experimental values obtained from pressure losses measurements. The friction numbers obtained from CFD calculations of the pressure losses in microchannels are in excellent agreement with both the experimental and the analytical results.