This paper presents the fracture behaviour of injection-moulded magnesium hydroxide filled polypropylene block copolymer (PP) as a function of the filler content, and it is compared to that of the unfilled PP. The J-integral concept was applied from tests carried out on SENB specimens according to the European Structural Integrity Society (ESIS) protocol for plastics. The results show lower fracture toughness (J(IC)) as the filler content increases in the composite, what is explained on the basis of morphological details.