Achieving an appreciable weight reduction in PP based composite materials, particularly the ones reinforced by glass fibers, is quite challenging while enhancing their mechanical properties and fullfilling other enviromental concerns. To address this issue, low density composites of Poly(propylene) (PP) and waste cellulose fibers (WCF) were produced by high-shear thermo-kinetic mixer. This technique facilitates the ease of processing for the mass production of such composite materials due to the availability of high shear rates and relatively short processing times during manufacturing. The structure-property behavior of the molded samples was investigated as a function of WCF content. Briefly, one-fold increase in elastic modulus, 18% increase in tensile strength, 87% increase in flexural modulus and 27% increase in flexural strength of PP were achieved by the addition of 30 wt.% WCF. The significant enhancements in mechanical properties were mainly attributed to the homogeneous dispersion of intrinsically stiff WCF filler in the PP matrix as a direct result of the high-shear mixing. These results mainly suggest that waste cellulose fibers can be used as an effective reinforcing agent in PP matrix instead of highly dense, non-renewable and non-biodegradable fibers, such as glass fibers, that prevents further stresses on the environment. Along with this, the reuse of waste cellulose fiber in PP matrix, particularly at high concentrations like 30 wt.%, evenly corresponds to the reduction of total PP consumption for PP based composite production. The main conclusion of the study is that the extensive blending technology gives us the ability to produce high performance thermoplastic based composite materials as well as addressing the world-wide waste disposal problem by reusing of natural wastes, which is a great opportunity to ensure sustainability and reduce enviromental and economical costs for many industries.