Polypropylene composites filled with glass beads (GBs) were prepared by means of a twin-screw extruder. The tensile properties and impact-fracture strength of the composites were measured at room temperature to identify the effects of the GB content and surface treatment on the mechanical properties. The results show that the relative elastic modulus increased nonlinearly, whereas the tensile strength decreased with increasing GB volume fraction (phi f). The notched impact strength increased with increasing phi f when phi f was less than 11%, and then, it decreased; this might have been related to the GB aggregation in the case of higher concentration. The mechanical properties of the composite systems in which the GB surface was treated with silane coupling agent were better than those of the composite systems filled with the untreated GBs under the same conditions. Furthermore, the impact-fractured surfaces were observed with a scanning electron microscope to understand the interfacial morphology between the inclusion and the matrix and to examine the toughening mechanisms. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012