The structure, crystallization, and thermal stability of polypropylene containing nanosized MCM-41 particle, whose pore channels were filled with polystyrene inside the pore channels with the aid of supercritical CO2, were investigated by X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses, respectively. The results of differential scanning calorimetry showed that the incorporation of MCM-41 (without template) increased the melting temperature and the crystallization temperature by approximately 7 and 1.8 degrees C, respectively. Although the incorporation of MCM-41 (with template) and MCM-41-S-PS decreased the melting temperature, the crystallization temperature and the crystallization rate increased. The result of crystallization kinetics showed dramatic increase in crystallization rate and decrease in t(1/2) and t(f) for PP/MCM-41 (without template) nanocomposite, whereas decrease in crystallization rate and increase in t1/2 and t(f) for PP/MCM-41 (with template) and PP/MCM-41-S-PS nanocomposites. The thermal results indicated that the thermal stability of the composite was enhanced by addition of all nanoparticles. At the 95% weight loss, adding 2.5 wt% nanomaterial, the decomposition temperature of PP/MCM-41-S-PS was 50.71 degrees C higher than that of PP, 13.98 degrees C higher than that of PP/MCM-41 (without template), and 0.39 degrees C higher than that of PP/MCM-41 (with template). POLYM. ENG. SCL, 49:2459-2466, 2009. (C) 2009 Society of Plastics Engineers