Abstract: The frictional resistances for wide side and narrow side of rectangular channels with different aspect ratios are compared, by analyzing the wall shear stress. The results show that the frictional resistance of the wide side increases sharply as the aspect ratio decrease, of which shows the reversed trend in a smaller range for the narrow side. Therefore the total frictional resistance increases with the decreasing of the aspect ratio. The frictional resistance proportion for wide side out of the total decreases as the aspect ratio increases, more slowly for the larger aspect ratio. The aspect ratio has remarkable influence on both the magnitude and the distribution type of shear stress for wide side, whereas, it has limited influence on shear stress for narrow side. Additionally, the transition Reynolds number of laminar to turbulent flow is calculated by energy gradient method and a polynomial correlation is achieved. The predicted results show good agreement with the experimental data.