The poly-(ethylene-b-propylene) was nucleated for a study of the structure modifications as well as the improvement of the mechanical properties, particularly the impact resistance. The nucleation was accomplished by two various red pigments: a linear trans-quinacridone dye (as beta-phase nucleator) and a dimethyl quinacridone resulting in the formation of a pure alpha-phase structure. Such procedure allowed the comparison of both the crystal structure and morphology of the copolymer. For the beta-nucleated copolymer a maximum of the k-value (fraction of beta-phase) was found for a concentration of the nucleating agent of 5 x 10(-4) wt.-%. By means of the WAXS measurement (in the transmission and reflection mode) an important structure gradient was detected in the cross-section of the injection moulded samples. This structure gradient was also observed by optical microscopy, where an agreement between optical observation of the spherulites and the k-value distribution on the cross-section of the samples was observed. For both nucleating agents (linear trans-quinacridone and dimethyl quinacridone) a dependence of the mechanical properties of the copolymer (modulus of elasticity and brittleness by impact resistance) on the pigment concentration was observed. For the beta-phase nucleated copolymer the maximum of the k-value of the hexagonal crystal structure corresponds to the highest ductility by impact test.