Abstract: The double-tube method was used to eliminate the influence of the pressure drop of inlet and outlet of microtube and the entrance effect on flow in microtube.Distilled water,tetrachloromethane and nitrogen gas,used as the working fluid,flowed through the quartz glass microtubes with inner diameters 19.60μm, 44.63μm,91.65μm and 140.86μm,respectively.The relation between friction factor and Reynolds number was studied by measuring the pressure drop and the flow rate.The experimental results show that the friction factors in microtubes,except the microtube with inner diameter of 19.60μm,are in rough agreement with those predicted by the Hagen-Poiseuille when Reynolds number is low.The friction factors depart from solution of Hagen-Poiseuille when Reynolds number is higher than 1300.For microtube with inner diameter 19.60μm,the values of the friction factor is 10%~15% higher.8%~10% less than those predicted by the Hagen-Poiseuille for the working fluids of distilled water,tetrachlormethane and nitrogen gas,respectively, when Reynolds number is less than 500.