In this study, paper processing residues containing a large extent cellulosic fibrous with inorganic filler such as calcite and clay minerals were used in the manufacturing of high density polyethylene (HDPE) composite. The residues ranging from 10 to 60wt% were loaded into HDPE in order to evaluate the effect of the paper processing residue filler on mechanical properties of HDPE. The composites were produced using mixing process by a high-speed thermo-kinetic mixer and molding process. Mechanical properties of the produced composites were analyzed using standard test methods. According to results, it was observed that the flexural modulus of composites increased with increasing filler contents. Especially, the flexural strength was increased by 26.3% for the 40wt% filler content The tensile strength of paper industry mill residues filled HDPE composites exhibited an increase of 10-20% compared to that of HDPE matrix. Consequently, in the present study, paper processing residues filler without any modifying agent was used for the production of HDPE composites having better mechanical properties compared to HDPE. Thus, paper processing residues which have no economic value and cause an environmental problem have served as a filler material for HDPE.