In this work, 3-(trimethoxysilyl)propyl methacrylate (MPTMS) was grafted onto polypropylene (PP) with use of dicumyl peroxide (DCP) as initiator in a twin screw extruder. Two batches of the grafted PP (PPgSIL) were prepared corresponding to two different grafting levels (4 and 8 wt.%). The presence of MPTMS in the PPgSIL samples was confirmed by FTIR, H-1 NMR, and elemental analysis. DSC and GPC results showed that the melting point(s) and molar mass of the PP were reduced due to the grafting. The two PPgSIL batches were used as coupling agents in viscose fiber/PP composites, each at three different levels (5, 2, and 1 wt.% of the weight of the composite). PPgSIL had a significant effect on the mechanical properties of the composites. The composites where PPgSIL was applied, the tensile strength increased by 59% relative to the composites without PPgSIL (64.6 MPa vs. 40.7 MPa). For comparison, the viscose fibers were treated with MPTMS in 1 wt.% MPTMS/water solution; however, the improvement in mechanical properties of the viscose fiber/PP composites was considerably less than that achieved with the PPgSIL coupling agent (46.6 MPa vs. 64.6 MPa). (c) 2007 Elsevier Ltd. All rights reserved.