The thermo-oxidative decomposition behavior of polypropylene (PP) and polyethylene (PE) filled with carbon nanotube (CNT) and organo montmorillonite (OMMT) as a heat stabilizing filler was comparatively investigated using nonisothermal- and isothermal–thermogravimetric (TG) analyses. The isoconversional method was employed to evaluate the kinetic parameters (E_a, lnA, and n) under dynamic heating in air. OMMT and CNT exhibited respective lowest and highest thermo-oxidative stability as revealed from the lowest and highest T_onset values, respectively, whereas thermo-oxidative stability of PE was better than PP. The TG profiles and calculated kinetic parameters strongly depended on the sorts of fillers and polyolefins. From TG data and calculated kinetic parameters, the incorporation of CNT and OMMT into PP and PE significantly improved the thermo-oxidative stability of the composites. The highest relative E_a value was observed for CNT-containing composites in the conversion extent region of 0.10–0.45. Simultaneous differential scanning calorimetry thermograms revealed that the degradation processes for the neat polyolefins and their composites were exothermic in air. By adding the same filler, the thermo-oxidative stability of the PE-containing composite system under isothermal heating at 320°C was much higher than that of PP composite systems. The obtained results of nonisothermal and isothermal investigations and morphological observation of the samples after heating suggested the improved thermo-oxidative stability of polyolefins modified by CNT over that by OMMT.