The processes of thermal and thermal-oxidative degradation taking place in polyethylene (PE) films modified by graft polyacrylic acid (PAA) have been studied by TG and DTA. The mutual influence of the components on the degradation of the macromolecules present in the graft copolymer has been revealed. The increase in thermal stability of PE in the graft copolymers is explained by the decrease in the molecular mobility in PE amorphous regions containing microphase particles of graft PAA.