Considering expected matter in calculation of thermodynamic properties of real fluids such as natural gas with molecular simulation, the accuracy of calculated values of thermodynamic properties and choice of potential model are discussed in this paper. The thermodynamic properties of argon and methane were calculated by the Monte Carlo method with Lennard-Jones 12-6 potential, and the calculated values were compared with the values tabulated by Rabinovich et al. and by Setzmann and Wagner, respectively. The accuracy of the density values is estimated to be better than ±1% in a wide range of temperatures and pressures. The behaviors of isothermal compressibility, vapor pressure, saturated liquid density and saturated vapor density are estimated to be reasonable, but that of isobaric expansivity is limited in a range of low densities up to the critical point.