Mechanical properties of polypropylene (PP) filled with wood flour was studied. To improve adhesion at the interface, different surface treatments of the fiber were carried out. Polymethylene polyphenylisocyanate (PMPPIC) and silane coupling agents (silane A-172, A-174, and A-1100) were used to pretreat the fiber before it was introduced into the polymer. Increase in tensile strength, with fiber concentration, was found in PP filled with PMPPIC pretreated wood flour. Elastic modulus was unaffected by fiber treatment. Izod-impact strength decreased with increased filler level in the composite.