The tensile behavior of high-density polyethylene (HDPE), polypropylene (PP), and linear low-density polyethylene composites containing a titanate coupling agent and silicone-oil-treated magnesium hydroxide [Mg(OH)(2)] was studied. The increase in the extent of the ultimate elongation of the composites was affected by the yield stress and the strain-hardening tendency of the polymer matrix in the composites. Ethylene-propylene-diene rubber and octane-ethylene copolymer were introduced to adjust the yield stress of PP and HDPE. Although the ultimate elongation of PP/elastomer and HDPE/elastomer blends was higher than that of virgin PP or HDPE, the ultimate elongation of the filled composites dropped at a high content of Mg(OH)(2). Scanning electron microscopy showed that the difference in the uniformity of the interface exfoliation decreased with the yield stress of the matrix. (C) 2003 Wiley Periodicals, Inc.