Polypropylene (PP) composites with filler of catalytic coke (CC) or fresh catalyst (FC) of Ni/SiO2 center dot MgO were prepared by melt mixing technique in a twin screw extruder. The catalytic coke, that has the fibril of carbon, is the waste from petrochemical industry and can be obtain cheaply in high volume. The composites were examined by assessing their mechanical properties, thermal properties, and the morphology of fracture surface of the PP matrix. The improvement of some mechanical properties of the CC filler such as Young's modulus and slightly improvement of tensile strength were observed whereas the elongation at break and toughness decreased with the increase in CC or FC. Both degradation temperature of CC or FC composites decreased with increased filler content but lesser in the case of CC. The structures of fracture surfaces were supported by the results from mechanical properties measurement. The CC composite in PP matrix (CCPP) exhibits the fibrils and the plastic deformation of the matrix, while the FC composite in PP matrix (FCPP) presents similar fracture roughness as the nascent PP. The catalytic coke can be utilized as the filler to improve the Young's modulus while the fresh catalyst cannot be utilized as filler because of the adverse effects. (c) 2008 Wiley Periodicals, Inc.