Abstract: A calculation formulation for laminar friction coefficient in narrow rectangular channel was obtained by theoretical analysis. Experiments have been made on flow characteristics in narrow rectangular channels with low aspect ratios, and the results show that the curve of friction coefficients in rectangular channel varying with Reynolds number is similar to that in round pipe. The conventional frictional correlations based on circular pipes are not available for rectangular channels, which depend on Reynolds numbers and aspect ratios, and the friction coefficient increases as the aspect ratio decreases. The experimental results agree well with the theoretical analysis. The effects of aspect ratios on frictional characteristics in laminar flow were explained by taking into account wetting perimeter and shear stress.