Experiments were conducted to investigate flow characteristics of water through trapezoidal silicon microchannels with a hydraulic diameter ranging from 51 to 169 mu m. In the experiments, the flow rate and pressure drop across the microchannels were measured at steady states. The experimental results were compared with the predictions from the conventional laminar flow theory. A significant difference between the experimental data and the theoretical predictions was found. Experimental results indicate that pressure gradient and flow friction in microchannels are higher than those given by the conventional laminar flow theory. The measured higher pressure gradient and flow friction maybe be due to the effect of surface roughness of the microchannels. A roughness-viscosity model is proposed to interpret the experimental data.