Sonic nozzle is widely used for reference of calibrating flowmeters in gas flow measurement and its use requires the critical flow factor on the thermodynamic properties of the gas at the nozzle throat. ISO-9300 provides the calculating method of the factor. But since the critical flow factors from this method show an errors over 0.5% in specific conditions and over 0.1% in general NG custody transfer condition, this method cannot be applied for gas flow measurement with sonic nozzle. Therefore in this study, the other method is used to calculated the critical flow factor for higher accuracy than the ISO method. The equation of state from AGA-8(''94) and high accuracy(within±0.05) model of ideal gas properties are used in calculating the factor. The results of this study show that the accuracy of critical flow factors are within ±0.1% in the NG custody transfer condition.