Fillers and reinforcing materials from natural resources offer various opportunities for usage in plastics technology. However, unlike synthetic fillers, they usually have no constant properties, but rather differ from each other, depending on harvest year, growing area or type. Furthermore, the influence of processing, e. g. during compounding, is very high because of significant factors such as temperature and shearing. In earlier studies it was shown, that there is an effect of filler size on the mechanical properties of composite materials, such as wood-plastic composites. Further influencing factors are filler geometry (aspect ratio, sphericity) and filler orientation inside the matrix, which are strongly induced by the processing like compounding and injection molding. Concerning the compounding process, a design of experiments was carried out with different processing parameters such as temperature, screw configuration, screw speed and mass throughput in order to examine the influence on the used filler itself and the mechanical properties of the produced composite materials. In this case, a twin-screw extruder was used to compound polypropylene with different types of wood particles. A maleic anhydride-polypropylene copolymer (MAPP) was used as a coupling agent to improve the adhesion between filler and matrix. Subsequently, the material was further processed by injection molding to test specimens for further investigations. Mechanical properties are represented by tensile, flexural and impact properties. The filler size and shape distribution in raw material, compound and finished component is determined using dynamic image analysis, X-ray microtomography and optical microscopy. A thermal analysis was also carried out to characterize the effects of the thermal load during processing.