Using old newsprint (ONP) fibers as reinforcing filler in polyester (PE) composite has been studied. Using ONP fibers in PE composite resulted in a decrease in modulus of rupture (MOR) and an increase in modulus of elasticity (MOE) and tensile strength as compared with a neat PE composite. Also, water absorption and thickness swelling were increased as a result of using ONP fibers in the composite. Acetylation, steaming, and esterification (using maleic anhydride) of ONP fibers were performed to improve the dimensional stability of the produced composite. Acetylation and steaming of ONP fibers resulted in a decrease in the thickness swelling of the produced composites; MOR, MOE, and tensile strength were also decreased as a result of these treatments. Esterification of ONP fibers using maleic anhydride resulted in a decrease in thickness swelling of the produced composite and, at the same time, an increase in MOR, MOE, and tensile strength.