Three natural fibers, named cellulose, sawdust and wheat straw were surface modified by triethoxysilane (KH-550), methacryloxy propyl tri-methoxy silane (KH-570), and maleic anhydride grafted polypropylene (MAPP); and then the influence of surface modification, fiber type and fiber content on the tensile properties of the polypropylene composites reinforced with these treated or untreated fibers. The results show that tensile strength of the composites tends to decrease with increasing fiber contents. MAPP treatment exhibited the best improvement for fiber-matrix adhesion compare to the silane coupling agent, with a maximum percentage increase of 50% in tensile strength; however, the breaking strain and breaking energy present an obvious decline due to the higher interface binding force.