The flow characteristics of the fluid in microchannels formed by different hydrophobic and hydrophilic walls on the polymeric microfluidic chips were researched in this study.To achieve the spontaneous capillary of the liquid in a microchannel, the aspect ratio condition of the microchannels for the smoothing capillary was proposed on the basis of the free energy minimization approach. The experiments of the capillary transportation in the microchannels with three Polydimelhysiloxane(PDMS) walls and one glass wall were performed to confirm the aspect ratio condition. According to proposed theory, the critical aspect ratio of the microchannels with the depths 165 μm, 200 μm and 265 μm is 0.5000. In our experiment, the critical aspect ratios are 0.4714，0.4878 and 0.4818 respectively.Obtain data show that the experimental results are in good agreement with the theoritical one,which verifies the aspect ratio obtained by our experiments.