This paper presents results obtained for the development of a wood-polymer composite (WPC) based on polypropylene (PP) reinforced by olive wood fibers (OWF). The effect of the wood flour content and its chemical fiber treatment (amino-silane) on the mechanical properties of the WPC was studied by ultrasonic methods and mechanical tensile test. The elastic properties of the studied PP/OWF compositions are discussed and both of the mentioned evaluations give similar tendencies even if the characterization methods are somewhat different. As a result, the increase of the fiber content and the addition of the amino-silane coupling agent is shown to improve the rigidity of composite materials. Eventually, a correlation factor between the estimated Young's moduli is established between the ultrasound values (for ε < 0.05%) and mechanical values (for 0.05 < ε < 0.25%). Ultrasound measurements are discussed as an alternative method for the elastic properties evaluation.