Semi-local friction factors of turbulent gas flow through rectangular microchannels were experimentally investigated in this paper. The experiments were carried out with two rectangular microchannels etched into silicon wafers and capped with glass. Their hydraulic diameters were 146.7 and 203.0 mu m. The stagnation pressure was selected in such a way that the flow is ranging up to Re approximate to 120,000 with the outlet faced to the atmosphere. The pressure was measured at six locations along the channel length to determine local values of Mach number, temperature and friction factor. Both the Fanning and the Darcy friction factors were obtained under the assumption of both a Fanno flow (adiabatic wall) and an isothermal flow, respectively. Supplementary data reduction of the measured data for D-h = 99.36 mu m and 146.76 mu m in the previous study (Hong et al., 2012) were carried out under the assumption of a Fanno flow. The effect of temperature decreases on friction factors were investigated because the gas temperature steeply decreases due to energy conversion from thermal energy into kinetic energy in a high speed microchannel gas flow. The obtained friction factors were compared with empirical correlations in the literature on Moody's chart. Fanning friction factors obtained under the assumption of a Fanno flow almost coincide with the Blasius correlation. The Fanning friction factors obtained under the assumption of an isothermal flow were lower than those obtained under the assumption of a Fanno flow because they did not include the effect of temperature decrease. (C) 2016 Elsevier Ltd. All rights reserved.