The effects of TiO2 particles on the crystallization and uniaxial stress–strain behavior of poly(ethyleneterephthalate) (PET) films from amorphous precursors were investigated. The addition of small fraction of sub micron sized TiO2 particles were found to suppress the mechanical relaxation processes associated with high temperature side of the β relaxation while enhancing the low temperature relaxation. When crystallized from unoriented precursors, TiO2 particles act as a nucleation agent and enhance the thermally induced crystallization of the PET chains. However, when stretched from the amorphous precursors in rubbery temperature range (Tg