This article presents the preparation and characterization of wood flour/polypropylene (PP) composites filled with graphene nanoplatelets (GNPs). The effects of GNPs, as reinforcing agent, on the mechanical and physical properties were also investigated. In order to increase the interphase adhesion, maleic anhydride grafted polypropylene (MAPP) was added as a coupling agent to all the composites studied. The morphology of the specimens was characterized using scanning electron microscopy (SEM) technique. The results of strength measurements indicated that when 0.8 wt.% GNPs were added, tensile and flexural properties reached their maximum values. At high levels of GNPs loading (3–5 wt.%), increased population of GNPs leads to agglomeration and stress transfer gets blocked. The addition of GNPs filler moderately increased the impact strength of composites. Addition of GNPs decreased the average water uptake and thickness swelling by 35% and 30%, respectively, compared to the control sample (without GNPs). It was observed that the composites filled with GNPs decomposed at higher temperatures compared to the pure PP and control. In all cases, the degradation temperatures shifted to higher values after the addition of GNPs. The improvement of physical and mechanical properties of composites confirmed that GNPs have good reinforcement and the optimum effect of GNPs was achieved at 0.8 wt.%.