The fracture behavior and deformation mechanism of polypropylene (PP)/ethylene-octene copolymer (POE)/magnesium hydroxide [Mg(OH)(2)] ternary composites were investigated. The fracture behavior of the PP/POE/Mg(OH)(2) ternary composites was strongly influenced by the POE content. In the case of brittle fracture, the debonding of filler particles dominated the deformation process. Strong shear yielding of the matrix ligaments between microvoids took place after the brittle-ductile transition. The existence of POE could improve the deformability of the PP matrix and change the debonding manner of the filler particles. The fracture behavior of the PP/POE composites was investigated by the single-edge notch tensile test at 1 mm/s (a low test speed). The introduction of POE led to a decrease in the crack initiation energy and an increase in the crack propagation energy. The improvement in the fracture energy with increasing POE content was dominated by the increase in the crack propagation energy. A morphology analysis of the PP/POE composites demonstrated that a mixture of separation and encapsulation microstructures existed in the matrix. (c) 2005 Wiley Periodicals, Inc.