Room-temperature ionic liquids (RTILs) are molten salts usually composed of relatively large organic cations and organic or inorganic anions which are liquid below 100 oC. An important advantage of ionic liquids is the tunable physical and chemical properties of this family. Some of their physical and chemical properties (e.g. density, viscosity, conductivity, melting point, solvation hydrophobicity, and hydrogen-bonding capability) can be tailored by the selection of suitable cations, anions, and constituent groups on the ions. The experimental study of ionic liquids is difficult due to large effects of common impurities such as water and chloride ions on the properties of these materials. Modeling and computational science could greatly aid in predicting properties and performance of ionic liquids. Due to importance and application of ionic liquids, we need a big data bank on the fundamental physical and chemical properties of ionic liquids, therefore predictive methods for estimation of thermophysical propertie in wide range of pressure and temperatures can be very helpful. The two most commonly used method of calculating PVT properties are using equations of state (EOS) and PVT correlation functions. The first requires adequate knowledge of molecular interactions which are rare and some cases unobtainable for complex fluids. This limitation will be intensified for ionic liquids, which are composed of a bulky, asymmetric organic cation (e.g. imidazole,…) and an anion of wide variety, ranging from simple inorganic ions (e.g. halides) to more complex organic species (e.g. triflate). The other point is that ionic liquids are composed of two different molecules (cations and anions) and nearly all the developed equations of state are for one unique and simple molecules. In this thesis, the compressibility factor and thermal pressure of some well know ionic liquids are calculated using five well known EOS and compared with experimental values. We showed that all of these EOS which are based o L. J. potential or its effective type are not able to predict these quantities accurately especially at regions with low compressibility factors, (i.e. . Therefore, we derived 4 correlation functions for the calculation of these properties. The first and second of these function (namely CF-1 and CF-2) can calculate only the compressibility factors, the third (namely CF-3) thermal pressure coefficient and the most important and practical correlation function is the fourth (namely CF-4) that can calculate both compressibility factor and thermal pressure coefficient simultaneously. Coefficients of thi PVT correlation are adjusted using experimental densities and thermal pressure of five ionic liquids based o imidazolium , [C2mim][NTF2], [C7mim][NTF2], [C8mim][NTF2], [C2mim][BF4], [C4mim][C(CN)3]. Predicted densitie and thermal pressures are in good agreement with experimental data in wide ranges of temperatures ( 293-393 K) and pressures (0.1-30 MPa). The mean percent deviation (MPD) is less than 0.5% for both densities and thermal pressure for these five ionic liquids. Although there are reported some correlation functions for ionic liquids, these correlation functions can predict only one property, (ie. Compressibility factor), while the merit of this correlation function (CF-4) is that it can predict both compressibility factor and thermal pressure accurately