PP/wood flour composites were prepared in a wide range of compositions in an internal mixer at 180°C, 50rpm and 15min. Interfacial adhesion was modified by the introduction of two maleinated polypropylenes (MAPP) with different molecular weights and functionality. Wood content was changed between 0 and 70wt%, while MAPP/wood ratio between 0 and 0.25. Compression molded plates of 1mm thickness were prepared for testing. Structure was characterized by X-ray diffraction and DSC measurements. Interfacial adhesion was estimated quantitatively with model calculations. The results showed that the morphology of the components and that of the composites does not change as a result of compounding and the wood flour used in these experiments does not nucleate PP. The most important structural phenomena determining properties are the orientation of the anisotropic wood flour particles and their aggregation. Both structural phenomena depend on processing conditions and on the dimensions of the product. Much larger improvement could be achieved by the addition of a functionalized polymer in thicker samples than in thin specimens. MAPP has a maximum efficiency at around 0.05–0.10 MAPP/wood ratio in the composites of this study due to the limited surface available for coupling. MAPP with larger molecular weight and smaller functionality proved to be more advantageous in the improvement of composite strength, because it can form larger number of entanglements per molecule than the small molecular weight compound. On the other hand, the smaller molecular weight coupling agent decreases viscosity and improves processability considerably, thus optimization of composition is essential to achieve maximum performance.