A product of friction factor and Reynolds number (f (.) Re) of gaseous flow in a quasi-fully developed region of a micro-tube was obtained numerically and experimentally. Two-dimensional compressible momentum and energy equations were solved for a wide range of Reynolds number and Mach number for both 'no heat conduction' and isothermal flow conditions. It was found from numerical results that the product of friction factor and Reynolds number (f (.) Re) in a quasi-fully developed region is expressed as a function of Mach number. The tube cutting method was adopted to obtain the pressure variation along the tube. Fused silica tubes of nominal diameter of 150 mu m, were used for experiments. The experimental results also indicate that (f (.) Re) is a function of Mach number. (c) 2005 Elsevier Ltd. All rights reserved.