The Enskog theory which is proposed for the hard-sphere potential can be modified for the real supercritical gases. The most well known modification which is proposed by Enskog is the modified Enskog theory (MET) which is used of thermal pressure instead of pressure. MET can not be applied for the whole temperature-density range and also needs to the thermal pressure and accurate virial coefficients of gases at specified density and temperature. In this thesis a new method for the calculation of viscosity of supercritical gases and refrigerants based o the Enskog theory for hard-sphere potential is proposed in which the diameter of the potential is taken as a function of temperature and density. For this reason this method is called effective hard-sphere Enskog method. It is shown that this method can calculate accurately the viscosity of different gases over a wide temperature-density range. The accuracy of this method in comparison with other well known methods for viscosity calculation of gases like MET and excess viscosity function is comparable and in some cases is better (about 0.5 percent), although the number of its adjustable parameters is less than the other methods. In the second part of this thesis, thermal expansivity and isothermal compressibility of some fluids (argon, methane and nitrogen) over a wide temperature-pressure range is calculated and fitted in precise correlation functions. It is possible to get these coefficients with high accuracy (about 0.06%) without cumbersome differentiation of complicated equation of states