Although liquids and dense fluids are complicated on a molecular scale, they show a number of simple regularities, some of which have been known for years without any theoretical basis. In 1993 Parsafar and Mason developed a new regularity for dense fluids and called it "Linear Isotherm Regularity", which is called in this thesis . The regularity is that the quantity is linear i , where is the molar density. is the compressibility factor and defined a , where is the molar volume, and are the gas constant and Kelvin temperature respectively. Further investigations showed that does show some deviations from experiment for extremely noherical molecules. It was also shown that versu isotherms for long-chain organic compounds show significant deviations from linearity. Moreover, cannot fit the data for the simple fluid neon if a wide density range is considered. Also, the application of to alkali metals, more specifically, liquid cesium, suffers within the density range where the metal-nonmetal transition occurs. In 2009, Parsafar and his coworkers developed a simple functional form for a general equation of state based on an effective near-neighbor interaction of an extended Lennard-Jones (12,6,3) type, and tested against experimental data for a wide variety of fluids and solids. Using arguments analogous to those of Parsafar and Mason (in deriving ) they obtained the equation of state, which is called in this thesis (because it builds on earlier linear regularities, and which perform well for particular materials). Three constants, and g are functions of temperature, and in general, all three functions to contain contributions from both the internal and thermal pressures. It gives an excellent representation of isotherms for widely differing materials and over wide ranges of density. In this thesis, the accuracy of the prediction of the compression factor of fluids (monatomic and polyatomic fluids, hydrocarbons and refrigerants) via EOS(III) is evaluated by calculation of ، و for each isotherms over a wide temperature-density range. This accuracy was also checked by using the temperature dependency ، and and shown that the level of the accuracy is not changed significantly. The main limitation of is the vast number of parameters of the equation of state, since for each fluid at each temperature, ، and must be determined. It is true that by using the temperature dependencies of و and , these parameters reduces to 12, but it must be accepted that working and determining this number of parameters is not an easy task. It was shown in this thesis that by using the principle of the corresponding states, it is possible to reduce this number of parameters from 12 to only 6 parameters. For this purpose, the plot of each ، and respect to temperature, was superimposed on the corresponding plot of the reference fluid (nitrogen gas) by using only two adjustable parameters. It was shown that the decrease of mean accuracy of predictions is not greater than 1 percent. Although can be regarded as the most reported accurate equation of state for dense fluids, this recipe of reducing the number of parameters give a better and more easy workable procedure for using this equation of state especially for practical and engineering purposes.