Polystyrene/nano-SiO2 composites with different silica contents have been prepared by using nano-silica particles of 10 nm and polystyrene obtained in batch jacketed reactor with self-tuning PID (STPID) temperature control. The relationship between final properties of nanocomposite material and polymer matrix properties with control has been studied and the composites thus prepared are characterized by means of differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), scanning electron microscope (SEM) and hardness tests. It is observed that higher mechanical and thermal properties of nanocomposites could be obtained using polystyrene produced with STPID temperature control under optimal conditions which are precalculated for average number of molecular weight of 52000 and monomer conversion of 50%. It is also found that hardness and thermal stability of nanocomposite materials increase with increasing nano-silica contents, as expected. The same thermal stability increase is observed in DSC thermograms.