The effects of size and concentration of wood particles on the properties of composites, obtained by extrusion, were evaluated based on polystyrene and wood particles from Eucalyptus globulus Labill. Wood-plastic ratios were 10:90, 30:70, and 50:50 (weight / weight), and wood particles were retained in 40, 50, 65, and 100-mesh sieves. The density, flow index, water absorption, and the mechanical properties were evaluated. Scanning electron microscopy revealed poor adhesion between the wood particles and the polystyrene. The size and content of wood particles were found to have a strong influence on the mechanical properties of the composite. The introduction of the wood particles induced a reduction of the Young’s modulus, ultimate strength and deflection, as well as an increment in the elongation at break. The impact resistance also increased with the size and concentration of the wood particles. Furthermore, with increasing content of wood particles, the value of the melt flow index decreased and the water absorption rose.