We have calculated some thermal properties of hydrogen and of oxygen using a classical molecular dynamics (MD) method. To examine the applicability of a simplified intermolecular interaction model, we employed the Lennard-Jones (12-6) potential with ignoring the rotational freedom in both fluids and conducted MD simulations for the Lennard-Jones fluid at a wide density-temperature range. Using the MD calculation data, we made a polynomial function of Helmholtz free energy, which can derive every thermal property, and determined the potential parameters of the Lennard-Jones model which reproduce the thermal properties of hydrogen and of oxygen. In spite of the very simple intermolecular interaction model, we found that it can reproduce the thermodynamic properties of oxygen at a wide density-temperature range and the pressure-volume-temperature relationship of hydrogen below its critical density. Such simple model has a potential to estimate reasonably an equation of state of hydrogen/oxygen mixture or the interfacial tension which is very important in the analyses of the coaxial injection jet flows of oxidizer with gaseous hydrogen in a rocket engine thrust chamber.