Abstract: The characteristics of the single-phase flow in narrow annular channels were analyzed and theoretical model was proposed. Based on the present model, the theoretical calculation was performed to predict the flow characteristics for the developed flow in narrow annuli with the gap sizes of 1.0, 1.5 and 2.0 mm, respectively. The calculation results are in good agreement with experimental data. In addition, the gap size of the narrow annuli has great impact upon the flow characteristics. The decrease of the gap size reduces the friction factor. The higher the Reynolds number, the larger the effect of the gap size upon the friction coefficient during single-phase flowing through narrow annular channels. The effect of the gap size upon the friction coefficient depends on the Reynolds number, and will decrease with the decrease of the Reynolds number.