The effects of different parameters (e.g. method of defibration of newspaper; size of the fiber; type and concentrations of coupling agents, impact modifiers, and fire retardants) on the mechanical properties of old newspaper fiber-filled recycled polypropylene (PP) have been evaluated. Statistical methods were applied to determine the effects of variables on the tensile strength, Young＇s modulus, tensile toughness, and impact strength. Statistical analysis revealed that the composites based on newspaper fiber prepared by mechanical defibration showed a better tensile strength compared with those prepared by the steam explosion process, while the latter type of fiber outperformed the former as far as the other mechanical properties are concerned. In the presence of maleic anhydride grafted PP (MPP) and the initiator dicumyl peroxide (DCPO), the tensile strength of the composites increased with increasing fiber loading and fiber size. Moreover, both the tensile strength and the modulus increased with an increase in the concentrations of MPP and DCPO, even in the presence of an impact modifier and fire retardants for surface-modified newspaper-filled composites. Since the tensile strength of short fiber reinforced composites is strongly dependent of the degree of adhesion between the fiber and matrix, the experimental results suggest that either one of these (MPP and DCPO) or both act as the interfacial bonding agent to develop a strong interphase between old newspaper fiber and recycled PP.