Abstract: This article utilized an IR camera to measure the temperature of outer wall of the micro steel tubes with inner diameter of 168 μm and 399 μm, and inner roughness of 8% and 9% respectively, through which distilled water flowed as the working fluid, under conditions of constant heating power and variant Reynolds number (Re). It also measured the pressure drop and the flux at the inlet and outlet of the micro steel tube, and calculated the Nusselt number (Nu) and frictional resistance coefficient (f) under different Re. The results show that when Re is lower, the f is in rough agreement with Hagen-Poiseuille solution, however, Nu is slightly higher than Hausen solution. When Re reaches at 800, f begins to depart from Hagen-Poiseuille solution, and Nu begins to increase quickly, which is due to earlier transition from laminar flow to turbulent flow.