A critical nozzle has been used to measure a mass flow rate of gas. It is well known that the coefficient of discharge of the flow in a critical nozzle is a single function of Reynolds number. The purpose of the present study is to investigate the effect of equation of state (EOS) on the coefficient of discharge and thermodynamics properties through the critical nozzle by using H-2 with the help of a CFD method. In computations of the flow field including the stagnation point upstream of the nozzle, the Redlich-Kwong, Lee-Kesler and Peng-Robinson equations of state were employed to take account of real gas effects. As a result of the present computations, coefficients of discharge using the Redlich-Kwong and Lee-Kesler EOS were in good agreement with experimental results in the range of high Reynolds number and the coefficient of discharge decreased with an increase of Reynolds number in the range of 1.0 x 10(5) < Re < 2.8 x 10(6). Crown Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.