This study aims to investigate the mechanical properties of a concrete mix reinforced with polyethylene terephthalate (PET) and polypropylene (PP) fibers. Various concentrations of fibers (0.5%, 1.0%, 1.5%, and 2.0%) of the total volume of fine aggregate were added to the concrete mix. The impact of these additions on compressive and flexural strength was evaluated and compared with the concrete mix without fibers. Compressive strength tests revealed an inverse correlation between the increase in PP and PET fibers and compressive strength. The lowest strength was for 0.5% PET and PP fibers in the mix. As for flexural tests, the mix with 0.5% PET and PP fibers showed the lowest rupture load and elasticity modulus. These results imply that PET and PP fibers in the concrete mix matrix preserve their adherence after failure. This study highlights the influence of the fibers on the mechanical properties of concrete, indicating their effect on compressive and flexural strength, as well as post-failure adherence of the mix. Keywords: Concrete Mix, Polyethylene Terephthalate, Polypropylene, Fibers DOI： https://doi.org/10.35741/issn.0258-2724.58.6.24