To evaluate the effect of coupling agents on the calcium carbonate (CaCO₃) filled high density polyethylene (HDPE) systems, three types of calcium carbonates- untreated, stearic acid coated, and isopropyl triisostearoyl titanate coated-were blended with HDPE by a twin screw extruder. Three compounded systems were investigated in terms of rheological properties, microstructure, and mechanical properties. At low frequencies, isopropyl triisostearoyl titanate reduced viscosity of the dispersed systems, while stearic acid increased the viscosity. Above the frequency of 20 rad/sec, however, the type of coupling agent had little effect on the viscosity, In general, two coupling agent treated systems increased impact strength and elongation at break but had a slight adverse effect on tensile strength. Examination of the fractured structure by SEM photomicrographs revealed that the two coupling agents might give an ability to form fibrils to the particulated system due probably to the enhanced compatibility between inorganic CaCO₃ and organic HDPE matrix.