Abstract: This research is aimed at determining the critical Reynolds number of the transition from laminar flow to turbulent in the microtube. The global pressure drop and velocity field were experimentally investigated for water flow in a glass microtube with a hydraulic diameter of 230μm and Reynolds numbers between 1020 and 3140. The axial mean velocity profiles and turbulent intensities obtained from Microscopic particle image velocimetry (micro-PIV) show that the transition from laminar flow to turbulent flow occurred for Reynolds number ranging between 1800 and 1900. This result is consistent with the global measurements of the pressure drop, and no obvious early transition to turbulence in the microtube is observed compared with the experimental data in the macro scale pipe flow. The experimental data of flow fields also suggest the flow was fully developed turbulent for Re>2660.