Change of physical property for dry heat treated DTY filament yarns was studied by investigating the change of molecular structure. Tensile strength, elastic recovery, and relaxation time roughly decreased with increase of heat treatment temperature. On the other hand, elongation and crystallinity increased with heat treatment temperature. PTT showed relatively high increase of crystallinity compared to PET. Minor increase of tenacity was found near $T_g$ of PTT. It was found that the elastic recovery is closely related to the number of Gaussian chain segment for rubber-like deformation. The relaxation time for U near 79 showed higher inclose than that of PET. In the estimation of relaxation time, fractional Maxwell model rather than Maxwell model simulated more reasonable value. Based on these results, it was deduced that physical cross-links such as entanglements of polymer chains formed when crystal is newly formed by the rearrangement of polymer chains.