Fundamental understanding of liquid flow through microchannels is important to the design and optimization of microfluidic devices. The objective of the current study is to experimentally investigate the effects of surface roughness on the flow characteristics in rectangular PDMS microchannel with a hydraulic diameter ranging from 66.67 to 200 μm. The experimental results are compared with the predictions from the conventional laminar flow theory. A significant difference between the experimental data and the theoretical predictions is found. Experimental results indicate that the pressure gradient and flow friction in microchannels are higher than those from the conventional laminar flow theory. This may be attributed to the fact that there exists the effect of surface roughness of the microchannels. The present results with a surface roughness model are compared with the experimental results to examine the validity of the model. It is found that there is a good agreement between the experimental data and the numerical predictions with a surface roughness model.