In this study, to understand how cellulosic materials characteristics influence the mechanical properties of polyvinyl chloride (PVC) composites, we first studied the effect of different types (wood flour and pulp fiber) and contents on selected mechanical properties of uncompatibilized wood plastic composites. We then compared the properties of hybrid composites to those of composites reinforced with wood flour or fiber, individually. PVC as polymer matrix and cellulosic materials were compounded by twin-screw extrusion and test specimens were prepared by injection molding. All tested properties vary significantly with filler form or content. With the addition of wood flour, the tensile strength moderately increases, but with the addition of hybrid filler and pulp fiber, it increases significantly. Notched impact strength increased with increasing particle size. Pulp fiber resulted in higher strength at the 40 wt % level compared with the strength properties of wood flour composites. The higher aspect ratio of the fiber had significant effect on the mechanical properties. Increasing filler load improves the strength of the composite up to a load of 40 wt %, further increase in the filler loading (> 40 wt %) results in a decrease in properties, due to the to filler agglomerates. Incorporation of 25% weight fraction of wood pulp fiber in hybrid composites gave the highest values of composite properties. The mechanical properties of hybrid composites are found to be much higher than that composites filled with wood flour. Finally, results showed that the hybrid composites had the maximum improvement in selected mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1788-1793, 2011