In recent years, the use of natural fibres as fillers in the manufacture of fibre reinforced thermoplastic composites has been of great interest, particularly in structural and automotive industry. New applications and end uses of wood-polymer composites (WPCs) for decking, flooring and outdoor facilities, window frames, various construction materials, must consider their exposure to atmosphere or contact with aqueous media. Poor resistance of the fibres to water absorption can have undesirable effects on mechanical properties and dimensional stability due to the embrittlement associated to the degradation of the macromolecular skeleton by hydrolysis. The most impressive enhancement of material properties was achieved using submicron-size fillers. Clay nanocomposites exhibit dramatic increases in modulus, strength, barrier properties, flammability resistance, heat resistance and hygroscopic (water absorption, thickness swelling) properties, compared with conventional composites. High and low molecular weight maleated polyethylenes (MAPE) were used in order to maximize their compatibilization effects on WPC/clay nanocomposites. Their results showed that the addition of organically modified montmorrillonite (OMMT) to a wood/polymer composite in the presence of MAPE reduces its linear coefficient of thermal expansion and increases the heat deflection temperature without affecting the processability [1,2]. Using carbon nanotube (CNT) in WPC based on PVC [3] yielded in composites with properties similar to solid wood. Our study is focused on the preparation of a new series of polypropylene/wood flour/montmorillonite (trade mark Cloisite 30B) by mixing and injection molding. Morphological investigation was performed by wide angle X-ray diffraction spectroscopy and transmission electron microscopy. The effect of the nanoscale filler on the long-term properties of the new nanocomposites was studied in terms of water uptake and mechanical properties. Along with the increasing organoclay content, the water absorption of the nanocomposites decreased. The intercalation morphology was observed for hybrid nanocomposite. The order of intercalation was higher for samples with low content of clay due to some agglomeration of organoclay that could appear in nanocomposites with high amount of organoclay. The mechanism of water transport in hybrid composites was Fickian diffusion and it seems that this phenomenon could be more restricted in exfoliated morphology through this mechanism.