Polystyrene/MCM–41 nanocomposites were synthesized by atom transfer radical polymerization (ATRP) at 110°C. Activators generated by electron transfer (AGET) and activators regenerated by electron transfer (ARGET), as two novel initiation techniques, for ATRP were used. Specific structure, surface area, particles size and their distribution and spongy and porous structure of the synthesized MCM–41 nanoparticles were evaluated using X–ray diffraction, nitrogen adsorption/desorption isotherm analysis, scanning and transmission electron microscopy images, respectively. The final monomer conversion was determined using gas chromatography. Number and weight average molecular weights (Mn and Mw) and polydispersity index (PDI) were also evaluated by gel permeationchromatography. According to the results, addition of 3 wt% MCM–41 nanoparticles into the polymerization media resulted in lowering conversion from 81 to 58% in the AGET ATRP system. Moreover, a reduction in the molecular weight of the products from 17116 to 12798 g/mol was also occurred, although, the polydispersity index increased from 1.24 to 1.58. The similar results were also obtained by ARGET ATRP system; lowering conversion from 69 to 43% and molecular weight from 14892 to 9297 g/mol, and an increase of PDI from 1.14 to 1.41. The improvement in thermal stability of the nanocomposites, as a result of higher MCM–41 nanoparticles loading, was confirmed by thermogravimetric analysis. In addition, according to the analytical results of differential scanning calorimetry, a decrease in glass transition temperature, due to the addition of 3 wt% of MCM–41 nanoparticles (from 100.1 to 91.5°C in AGET ATRP system and from 100.3 to 85.8°C in ARGET ATRP), was achieved.