Four different types of high-density polyethylenes (HDPE) were blended with perlite at different concentrations. Silane coupling agent gamma-aminopropyltriethoxy silane (gamma-APS, A-1100) was used to enhance the adhesion between perlite and HDPEs. Ultimate tensile strength and elastic modulus increased as the perlite content increased, while ultimate elongations decreased with the increasing amount of perlite. Exceptional variations in the measured properties are explained in terms of the differences in polyethylenes used in the composites. The biggest improvement in ultimate tensile strength was observed in the highly crystalline polyethylene; on the other hand, in the absence of silane coupling agent the high molecular weight polyethylene showed the least improvement in tensile strength. The effect of branching in HDPE composites was demonstrated. The enhancement of interfacial adhesion by using a coupling agent was also examined by scanning electron microscopy (SEM).