Nitrogen flow characteristics in Polyetheretherketone microtubes with inner diameters (D) ranging from 0.255mm to 0.553mm were experimentally investigated. It is indicated that most of the experimental points in laminar region are coincided with the conventional theoretical predicted value, but several plots caused by instrumental errors are lower than predicted values at small Re. In turbulent region, the friction factors for D = 0.255mm microtubes with L = 0.800m and 1.591m are slightly lower than conventional values; the experimental data for D = 0.553mm microtube with L = 0.800m is lower than that in D = 0.255mm pipes. The entrance effect obviously influences friction factor even if the L/D of microtubes is more than 60, where it can always be neglected in macro-scale. Due to the enhancement of compressibility effect as diameter decrease (Kn increase), friction constant is larger in smaller-size microtubes. The transition Reynolds number in current experiment (except for L = 0.200mm and D = 0.553mm) ranges from1600–2000, while a little early transition phenomenon is found in L = 0.200m, D = 0.553mm tube.

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