This study investigated the effect of plasticizer polyisobutene (PIB) of molecular masses ranging from 480 to 1.600 g/mol in isotactic polypropylene homopolymer. Compositions containing up to 7% of PIB were prepared in an internal mixer and samples were produced by compression moulding. Experiments of tensile testing, scanning electron microscopy, X-ray diffraction and differential scanning calorimetry were conducted with the polymer and with the mixtures. The results of melt flow index indicated a pronounced reduction in viscosity when the plasticizers were present. The plasticizing effect of PIB was also observed in the mechanical tests, with a drop in tensile strength and elastic modulus. X-ray diffraction showed a decrease in PP´s crystallinity. The micrographs obtained by SEM did not reveal phase separation, confirming the miscibility of the components used.