Polypropylene-low density polyethylene (PP-LDPE) blends involving PP-LDPE (90/10 wt%.) with (0.06 wt%) dialkyl peroxide (DAP) and different amounts (5, 10, 20 wt%) of calcium carbonate (CaCO3) were prepared by melt-blending with a single-screw extruder. The effect of addition of CaCO3 on thermal decomposition process and kinetic parameters, such as activation energy and pre-exponential factor of PP-LDPE blend with DAP matrix, was studied. The kinetics of the thermal degradation of composites was investigated by thermogravimetric analysis in dynamic nitrogen atmosphere at different heating rates. TG curves showed that the thermal decomposition of composites occurred in one weight-loss stage. The apparent activation energies of thermal decomposition for composites, as determined by the Tang method (TM), the Kissinger-Akahira-Sunose method (KAS), the Flynn-Wall-Ozawa method (FM)), and the Coats-Redfern (CR) method were 156.6, 156.0, 159.8, and 167.7 KJ.mol(-1) for the thermal decomposition of composite with 5 wt% CaCO3, 191.5, 190.8, 193.1, and 196.8 kJ.mol(-1) for the thermal decomposition of composite with 10 wt% CaCO3, and 206.3, 206.1, 207.5, and 203.8 kJ mol(-1) for the thermal decomposition of composite with 20 wt% CaCO3, respectively. The most likely decomposition process for weight-loss stages of composites with CaCO3 content 5 and 10 wt% was an A sigmoidal type. However, the most likely decomposition process for composite with CaCO3 content 20 wt% was an R-n contracted geometry shape type in terms of the CR and master plots results. It was also found that the thermal stability, activation energy, and thermal decomposition process were changed with the increase in the CaCO3 filler weight in composite structure. Copyright (C) 2009 John Wiley & Sons, Ltd.