The effects of the addition of interface modifiers on the structure and properties of wood fiber-filled polypropylene (PP) have been studied. Although an increase in the wood fiber loading reduces the overall cost of composites, the mechanical properties of such composites are very poor in the absence of a suitable interface modifier. The poor mechanical and thermal properties of the unmodified composite are attributed to increased heterogeneity induced in the system with increased concentration of wood fiber. A significant improvement in tensile strength and thermal resistance of these composites in the presence of a suitable interface modifier such as maleated polypropylene, itaconic anhydride, or bismaleimide-modified polypropylene is due to the improved dispersity of wood fibers in the polypropylene matrix as well as to the development of a stable interface effected by chemical interaction. In general, all modifiers are found to affect the crystallinity of PP in composites, and an improvement of tensile strength is marked by a lowering of crystallinity of PP.