Abstract: Experimental studies of single-phase flow characteristics in vertical and inclined rectangular channels with different aspect ratios were carried out. The experimental results indicate that the flow transition occurs when Reynolds numbers are larger than 2 400. In laminar state, the friction factors of both upright and acclivitous rectangular channels are larger than that of the classical laminar solution of round channels. In turbulent area, the friction factors are shown to be in good agreement with the Blasius expression. The inclinations have no effect on difference on the single-phase flow characteristics in narrow rectangular channels, but the friction factors increase with the decrease of aspect ratios.