The thermal diffusion factor for binary mixtures of hard spheres was calculated by a direct non-equilibrium molecular dynamics simulation method. The results obtained showed that for equimolar isotopic binary mixtures of hard spheres, the thermal diffusion factor (TDF) increases with density at fixed mass ratios and increases with mass ratio at fixed packing fractions. The dependence of TDF on diameter ratio was also investigated and it was found that if the mass ratio is equal to 1 then larger species are accumulated in the warm region. The reciprocal of the thermal diffusion factor as a function of the mole fraction of the heavier component in two packing fractions was studied as well. The results obtained showed that inverse of thermal diffusion factor 1/TDF tends to be more linear when the mass ratio significantly differ from 1 and the diameter ratio is close to 1. These results are in good agreement with revised Enskog theory (RET). It was also found that those low density results continue to apply at higher densities too.
