X-ray diffraction methods and polarized optical microscopy have been used to investigate the structural change of syndiotactic polystyrene/clay nanocomposites. The nanocomposite has prepared by mixing an sPS polymer solution with organically modified montmorillonite. Both X-ray diffraction and transmission electron microscopy results indicate that most of the swellable silicate layers are exfoliated and randomly dispersed into the sPS matrix. The X-ray diffraction data also show the presence of polymorphism in sPS/clay nanocomposites, which is strongly dependent on the thermal history of the nanocomposites from the melt and on the content of clay. In this study, the effect of premelting temperatures and crystallization temperatures of sPS and sPS/clay nanocomposites on their crystalline phases is discussed.