Dynamic modulus of elasticity (MoE) and shear modulus of wood-filled polypropylene composite at various filler contents ranging from 10% to 50% was determined from the vibration frequencies of disc-shaped specimens. Wood filler was used in both fiber form (pulp) and powder form (wood flour). A novel compatibilizer, m-isopropenyl-alpha,alpha-dimethylbenzyl-isocyanate(m-TMI) grafted polypropylene with isocyanate functional group was used to prepare the composites. A linear increase in dynamic MoE, shear modulus, and density of the composite was observed with the increasing filler content. Between the two fillers, wood fiber filled composites exhibited slightly better properties. At 50% filler loading, dynamic MoE of the wood fiber filled composite was 97% higher than that of unfilled polypropylene. Halpin-Tsai model equation was used to describe the changes in the composite modulus with the increasing filler content. The continuous improvement in elastic properties of the composites with the increasing wood filler is attributed to the effective reinforcement of low-modulus polypropylene matrix with the high-modulus wood filler. (c) 2006 Wiley Periodicals, Inc.