Thermal stability of isotactic polypropylene with or without calcium carbonate nanoparticles (nCaCO3) was investigated by chemiluminescence under isothermal regime at 190 °C. Two kinds of nCaCO3 particles, i.e., (neat and stearic acid-coated ones) were used. The contents of nCaCO3 within the iPP/nCaCO3 nanocomposites were 5, 10, 15, 20, and 25% w/w. Several parameters, i.e., oxidation induction time, oxidation half-time, maximum oxidation time, and oxidation rate were used to quantify the thermal stability of both the neat and the nCaCO3-filled iPP systems. The contribution of nanoparticles on the progress of oxidation is discussed. It has been found that the concentration of nCaCO3 increases the stability of systems when nanoparticles were covered, while the filler consisted of unmodified particles, the decrease in thermal strength with the increase in filler concentration was noticed.