The well-dispersed core-shell nanospheres (TiO2-g-PS) were synthesized via nano-TiO2 grafting polymerization and were used to prepare PS/TiO2 nanocomposites by melt blending. The tensile and thermal properties of the nanocomposites were measured by means of tensile test and thermogravimetric analysis (TGA). The fracture surface of the tensile test specimens was also explored through scanning electron microscopy (SEM). The reasults show that the tensile performance and thermal stability of the PS/TiO2 nanocomposites depend on the loading and state of the TiO2 in the PS matrix. The optimal properties of nanocomposites are achieved at 1.0wt% of TiO2-g-PS nanoparticles and the tensile strength is increased up to 43.6% comparing to the pure PS. The onset temperature at which 10% mass is lost is increased 10oC. The dispersion of the particles and interfacial adhesion lead to enhance these properties.