Samples of the blow-moulding grade HDPE filled with Kaolin were characterized by wide-angle X-ray scattering, microhardness and stretching calorimetry techniques. It is shown that crystallinity of the polymer matrix in the filled samples remains essentially the same as that in the neat polymer regardless of the filler content. The thermoelastic behaviour of all samples below the apparent yield point epsilon* is quantitatively described by classical equations for elastic solids. The thermoelastic parameters of the boundary interphase (BI) are discussed in terms of predictions of the step-by-step averaging approach. The experimental values of the internal energy increment in the inelastic strain interval for unfilled and filled samples are analyzed in the light of the filler debonding processes in the latter.