This study is aimed at utilizing the gramineae to reinforced polyethylene (PE). The interface modification was performed by treatment of silvergrass (SV) fibers with sodium hydroxide (NaOH) and polymeric methylene diphenyldiisocyanate (PMDI). The modified fibers were characterized by Fourier transform infrared spectroscopy. Composites were fabricated with different fiber loadings (10%, 20%, 30%, 40%, 50%, and 60%) of SV fibers by injection molding. The properties of the composites were studied by mechanical property, thermogravimetric analysis, and differential scanning calorimetry. The mechanical properties of the treated composites were compared well with those of untreated composites. A marked improvement of 49.0% in tensile strength and 47.35% in flexural strength for 40% SV fibers-reinforced high-density PE composites was noticed. It is also found that the treated fiber is acting as a nucleating agent. It can be deduced that the thermal stability of the wood–plastic composites can be improved when SV fibers were treated by NaOH and PMDI.