The purpose of this study was to obtain the data on the influence of fiber from the products of recycled polyethylene terephthalate (PET) on the strength properties of fiber-reinforced concrete. In the experiments, used the direct product of industrial recycling of plastic bottles (flex), as well as specially prepared smooth and ribbed fiber from the same raw material. The influence factors were chosen fiber length from 1 to 5 cm and the amount of fiber-from 1 to 3 % by weight of cement. It has been found that the use of PET fibers provides a gain in the concrete tensile strength of up to 66 %. The compressive strength of the studied samples of fiber-reinforced concrete within the limits of the varying factors decreased by 3–25 %. The most favorable ratio of the increase in the bending tensile strength and the decrease in the compressive strength was obtained in concrete with the addition of smooth 4cm long fiber with the smallest reinforcement value. The abrasion capacity (abrasive wear) of the samples decreased with the addition of 3 % of smooth and ribbed 4 cm long fiber. The research resulted in the experimental determination of the effective fiber length (critical fiber length) providing the most reliable fastening of the fiber in the concrete matrix.