Abstract: The characteristics of liquid flow in the microchannels with circular and trapezoidal cross-sections were studied by numerical simulation. The friction factor of liquid flow in microchannels was investigated with different pressures, the sizes and roughness of microchannels. The different flow regimes of laminar and turbulence were also analyzed in the microtubes. In addition, the range of a transition Reynolds number can be inferred from the friction factor profiles under different Reynolds numbers. The friction factors in the microchannels decrease with the increasing of Reynolds numbers gradually. The Reynolds number at transition from laminar to turbulent flow changes with the cross-section’s equivalent diameter. At the same Reynolds number, the friction factor increases with the roughness of the inner wall of the microchannel when the flow is turbulent.