Abstract: Accurate definition of the compressibility factors of natural gas is required for both the calculation of 2-phase flow of crude oil and gas, and the calculation of process design of natural gas storage and transportation as well as processing. Yet, defining the factors only by consulting Standing-Kate's Corelation Diagram can not meet the requirements for computer programming. The comments below are obtained through comparison between Standing- Kate's Corelation Diagram and the calculated results got through the five conventional methods of calculating the compressibility factors of natural gas and through evaluation of the five relative Corelation equations; 1. The relative curve between Dranchuk-Purvis-Robinson Method and Hall-Yarborough Method tallies greatly with Standing-Kate's Corelation Diagram about the compressibility factors of natural gas, with high calculation accuracy under all kinds of Corelation pressures and temperatures; 2. Under the condition of T_r < 2.6, Brill-Begg's Method tallies well with Standing-Kate's Corelation Diagram of the compressibility factors of natural gas, again with high calculation accuracy and wide range of applicability; 3. Gopal Method is recommended for manual calculation.