Impact fracture strength is an important characterization for impact toughness of materials. A polypropylene (PP) filled with diatomite with different diameter (5, 7, and 13 mu m) was fabricated by means a twin-screw extruder. The impact fracture strength of these composites was measured at room temperature to identify the effects of diatomite content and diameter on impact fracture strength of filled polypropylene composites. The results showed that the influence of diatomite on the notched impact strength was significant. When the volume fraction of the diatomite (phi(f)) was less than 10%, the notched impact strength (sigma(f)) increased quickly with an addition of phi(f), and then the variation of sigma(f) was slight. The notched impact strength of the composite with the diatomite diameter of 7 mu m is the highest when phi(f) was 10%. Furthermore, the impact fracture surface was observed by using a scanning electronic microscope (SEM) to study the toughening mechanisms. POLYM. ENG. SCI., 49:1603-1607, 2009. (C) 2009 Society of Plastics Engineers