Redlich-Kwong equation is a sort of model of real gas. To the unpolar gas,for example hydrocarbon,its definition is higher than Van De Walls equation. Thermodynamic theory and Redlich-Kwong gas' state equation show that the intrinsic energy,entropy and thermal capacity are all functions of temperature and volume,and their Redlich-Kwong gas' critical compressibility factors are at1.2824.