Wood-plastic composites (WPC), made of wood fiber (WF) and thermoplastics, have been well developed and commercialized in construction, packaging, automotive, and furniture industry over the past three decades, as they are environmentally friendly and require less maintenance compared with natural wood. Polypropylene (PP) is one of major thermoplastics used in WPC. However, the relatively poor ... [Show full abstract] mechanical properties of WPC prevent them from penetrating into other structural applications where strong mechanical properties are required. To enhance the mechanical properties of PP/WF composites, this paper applies one-step direct injection molding to produce the hybrid WPC with glass fiber (GF) and carbon fiber (CF). Compared with those of PP/WF composites, the tensile strength and tensile modulus of PP/WF/GF hybrid composites increased 30% and 26%, respectively, and the tensile strength and tensile modulus of PP/WF/CF hybrid composites increased 38% and 78%, respectively. Apart from the significant increase of mechanical properties, the additional benefit of hybrid WPC was that their flame-retarding property was improved. Particularly, the dripping behavior (or fire spreading) during burning associated with WPC was significantly reduced or eliminated for the hybrid composites. In addition, the water absorption and the surface roughness of hybrid composites were also studied. This study demonstrates that it is feasible to make hybrid WPC with one-step injection molding, and hybrid WPC open the door to their potential structural applications.