Abstract: Nitrogen gas,used as a working fluid,flows through a micro steel tube with inner diameter 168 μm and outer diameter 406 μm.Using directly electrical heating method,the outer wall temperature fields of the micro steel tube are acquired by employing an IR Camera and specially magnifying lens for constant flow rate, different heating power and constant heating power,different flow rate,respectively.The flux and the temperature of the inlet and the outlet of the micro steel tube are measured synchronously.A correlation between the axial conductive heat and the convective heat transfer is obtained by a theoretical analysis based on the experimental results.Investigations clearly show that the axial conductive heat weakens the convective heat transfer in micro steel tube and the decrement may be more than 2% compared to the quantity of whole convective heat transfer when the working fluid is nitrogen gas.