Melting peaks and resulted crystal forms in syndiotactic polystyrene (s-PS) subjected to various thermal treatments were thoroughly investigated using DSC and X-ray diffraction. This study concluded that the four major crystal forms were individually generated and no transformation occurred between alpha', alpha ", beta' and beta " forms during thermal treatments. Thickening of these crystals, however, was likely during either slower heating scan (i.e. dynamic annealing) or extended isothermal annealing at relatively high temperatures. Different polymorphs were found to dominate the observed multiple melting endotherms of a thermally treated s-PS sample, which was erased of the thermal history in advance. Through cross-reference between the thermograms and X-ray crystallographs, each of the multiple melting peaks in DSC thermograms of s-PS was individually assigned and associated with the identified crystal forms. The melting peak of beta-form is always located at lower temperature than those of alpha-form. Meanwhile, beta-form is less thermally stable than alpha-form.