Modification of high-density polyethylene with binary mixtures of ɛ-caprolactone and ɛ-caprolactam was performed. The composition-property diagrams reflecting the dependences of the physicomechanical, rheological, and optical properties of the resulting polymeric compounds on the content of modifying mixture components were constructed, and the thermal oxidative degradation of the blend samples was studied. Based on the results obtained, the optimal modifier concentrations were suggested.