This work focuses on the modification of the tensile yield strength of CaCO3 filled HDPE brought about by the incorporation of SEES elastomers. Two types of SEES elastomers were used, grafted and ungrafted with maleic anhydride functions. The grafted elastomer encapsulates the filler particles in-situ and creates an adhesive interphase. The tensile yield stress was increased with increasing content of grafted elastomer until a maximum value. The influence of the interfacial area and the volume fraction of filler were studied. It was shown that the relative increase in tensile yield stress when increasing amount of interfacial agent was added, both depends on the volume of filler and the interfacial area.