The aim of this study was to investigate the polyvinyl alcohol effect on the fundamental properties of WPC by the measurement of mechanical and physical properties, water absorption and swelling and effect of UV radiation. In this research WPC material were prepared using three different fractions of birch (Betula) wood flour and Bleached-Chemi-Thermo-Mechanical aspen (Populus tremula) pulp (Aspen BCTMP). Wood flour and BCTMP was modified with polyvinyl alcohol (PVA) and 3-aminopropyltriethoxysilane (APTES). Composites were made using two different thermoplastic polymers, polypropylene (PP) and maleic-anhydride-grafted linear low-density polyethylene (LLDPE-g-MAH) as a matrix material. Composite samples were prepared by injection molding. WPC samples were treated with accelerated weathering tests and mechanical properties were investigated using three-point bending test and Charpy impact test. It was found that PVA coupling increased the mechanical and physical properties of WPCs compared to the untreated wood filler based composites. Mechanical tests showed that smaller wood flour fraction size gives better flexural properties than larger fraction size, because smaller wood fraction size was more evenly distributed in the composite. Also, decrease of impact strength was noticed while adding wood flour to the thermoplastic polymers making WPC more rigid and brittle. The best WPC material properties were acquired when using maleic-anhydride-grafted linear low-density polyethylene (LLDPE-g-MAH) and wood flour modified with APTES was used. However, PVA cross-linked composites propproperties were only slightly lower. Water absorption and swelling of the composites decreased the flexural modulus and increased impact strength resulting weaker material with increasing the deflection of WPC. However, UV radiation also decreased composites flexural strength and modulus, and increased impact strength.There were color changes and cracks found on the surface of WPC materials.