Recycled Kraft fibers coming from old sacs were used for the preparation of polypropylene (PP) composites. Composites up to 50 wt.% of reinforcement were obtained and maleated polypropylene (MAH-PP) was used as coupling agent for improving the fiber-matrix compatibility and adhesion. In the present work, the mechanical properties of the obtained composites as well as the evaluation of interfacial behavior are studied. The results discussion is based on the analysis of the mechanical properties of composites, measurements of surface polarity, specific surface, fiber dimensions, spectroscopic and microscopic analysis. Different adhesion mechanisms at fiber-matrix interface must be presumed depending on the existence of MAH-PP coupling agent, as revealed the analysis of the modification reaction and the conducted proofs. The results showed that the addition of maleated polypropylene favored the incorporation of the relatively high amounts of this natural reinforcement and promoted the stress transfer inside the composite. Thus, an increase up to 50 wt.% of recycled Kraft fibers increased two times the ultimate tensile strength of the non-reinforced matrix. Additionally, the intrinsic mechanical properties of recycled Kraft fibers are deduced by means of modified rule of mixtures and compared to those available in the literature. Finally, an approach of the industrial competitiveness of these fibers, daily present in the market, with respect to E fiberglass is carried out. For this reason a comparison between the mechanical properties and the final cost of PP composites from recycled Kraft fibers or E fiberglass has been considered. (C) 2007 Elsevier B.V. All rights reserved.