The effective utilization of lignin waste has been a main concern in the bio-ethanol industry which uses straw and stalks as feedstock. In this study the lignin waste was incorporated into high density polyethylene (HDPE) to make composites by extruded pellet fabrication and thermoform process. With the increasing of the lignin content from 55% to 75%, the static flexural strength of the resulting composites was hardly influenced, but the tensile strength decreased 42%. Even though it was, the mechanical properties of lignin/HDPE composites were superior to those of traditional medium density fiberboard (MDF). Scanning electron microscopy showed that addition of 4% MAPE promoted the interfacial bonding between lignin and HDPE. Therefore, the macro performance also shows an improvement to mechanical properties.