Abstract: Experimental investigations of water flow friction in horizontal annuli,with gap sizes of 0.540～2.685 mm,were carried out to determine the effect of gap sizes on the flow resistance characteristics,size is above 2.5 mm,flow resistance characteristics accord with normal size annuli.The flow resistance coef-and the test involved the laminar and turbulent flow regimes.The results showed that of the characteristics of the fluid friction resistance of small scale annuli were obviously different with the theoretical results.The critical Reynolds number in small-scale annuli was slightly less than in normal size annul,but when the gap ficient formula of narrow annuli were established based on the experiment results.