Abstract: A calculation method of gas fugacity,traditional a function method,has been discussed on its complexity and error.The error of a function method consists in the inequality between its value and that value when the pressure is close to zero. The substitution of Z function for a function, it shows that the deficiency of a function can be overcomed. The conditions and range of applications of Z function are also explained.One of the advantages is to estimate the field of pressure that obeies the perfect gas law so that it is used widely in practice.