The efficacy of defective sites in MWCNTs as free radical scavengers was investigated on the basis of thermalanalysis of ultra-high molecular weight polyethylene (UHMWPE)/MWCNTs and UHMWPE/$\gamma$-MWCNTs nanocomposites. Homogenized composites of polyethylene (PE) with MWCNTs and $\gamma$-MWCNTs (0.5% by weight) were prepared by the ball milling process for 2 h at a speed of 200 rpm. Thereafter, micron-sized sheets were prepared (by hot pressing) and subjected to $\gamma$-dose ranging from 25 to 150 kGy. To investigate the thermal properties and thermal stability, differential scanning calorimetric (DSC) and thermo-gravimetric analysis (TGA) measurements were performed in an inert atmosphere. Theresults showed that peak melting temperature ($T_{\rm m}$) and lamellae thickness ($L_{\rm c}$) remained unaltered for UHMWPE/MWCNTs composites; however, for UHMWPE/$\gamma$-MWCNTs composites the values varied from 138 to 141$^{\circ}$C. This behaviour was attributed to chain scission close to crystalline lamellae due to radiation-induced free radicals, thus reducing the percentage of inter-phase contents within the polymer matrix and enhancing the percentage crystallinity for composites. Furthermore, TGA revealed the higher thermal stability of composites as compared with pristine ones and significant increase in residues percentage for composites, i.e., from $\sim$3 to 7%. These results confirmed the importance of defective sites within the MWCNTs as free radical quenchers, and stopping the chain scission, particularly close to crystalline lamellae, thus preventing loss of the important interphase region of UHMWPE.