Abstract: Equations of state (Eos) are usually used in oil and gas production and transportation. Eos are generally semi-empirical or semi-theoretical expressions; therefore, they have some application ranges. While Eos are adopted to calculate thermodynamic parameters of pure gas or non-polarity molecules, they have good agreement with field data, while Eos are used to calculate polarity molecules, they have bad agreement with field data. SRK, RK, PR, PT, SHMWR and LHSS Eos are often used to calculate the field data, such as compressibility factors of gas and liquid, fagacity of gas and liquid, and gas fraction which is using flash calculation. As result of calculation, PR, PT and LHSS Eos are better than others, and calculation values of SRK, RK, SHBWR Eos are much less than PR, PT and LHSS.