Free volume theory has successfully predicted and described all of the liquid-to-solid transitions in hard sphere system. However, in polymer system, the exact molecular picture of what is "free volume" is still vague. The only experimental observation which may be related to the molecular packing density and possibly the "free volume" is the small change in the scattering function, S(q) (or the pair correlation function), at q-1 in the range of monomer dimension. We propose to use NIMROD and inverse molecular dynamics Simulation to study the free volume in polystyrene (PS) bulk above and below its glass transition temperature to reveal the existence of free volume. As a direct quantitative evidence of free volume in polymer bulk, it can give more insight into the nature of glass transition.