In an industrial process for the design and simulation, the phase equilibrium of mixture and thermodynamic properties are required. Due to this reason, the flash calculation plays an important role to solve the industrial problem. In this research, the Peng-Robinson Equation of the state is applied which is widely used to find out the thermodynamic properties at phase equilibrium in the critical region for the system of hydrocarbon and related compounds. The behavior of four binary mixtures is observed by using mathematical modeling of Peng-Robinson Equation of the state. The pressure-volume behavior and pressure mole fraction behavior of both liquid and vapor phase at equilibrium are studied by mathematical modeling of isothermal flash calculation. The pressure mole fraction behavior is compared with the experimental data taken from literature which states that both mathematical modeling’s of isothermal flash calculations flow same behavior with the experimental data.