Isotactic polypropylene (iPP) was nucleated specifically for structural and mechanical studies, especially of impact resistance. Nucleation was accomplished by two 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. This procedure allowed the comparison of both the crystal structure and the morphology of the polymer. For the beta-nucleated iPP, by means of the WAXS measurements in the transmission mode, a maximum of the k-value (fraction of beta-phase) was found for a concentration of 5 x 10(-5) wt% of the trans-quinacridone. By means of polarized light microscopy (PLM), the mold temperature and pigment concentration dependent spherulites distribution on the cross section of the samples was observed. DSC permitted confirmation of the WAXS measurements,giving a characteristic double melting peak for the beta polypropylene. By isothermal WAXS measurements the temperature of 135 degrees C as the upper limit of the beta phase was detected. For both nucleating agents (linear trans-quinacridone and dimethyl quinacridone) the dependence of the mechanical properties (modulus of elasticity and brittleness by impact resistance) on the pigment concentration was observed. For the beta-phase iPP the highest ductility by impact testing corresponds eo the highest values of the k parameter. The explanation is based on the SEM observations, which permitted the detection of spherulites with cross-hatched lamellae (for the alpha-spherulites) and with simple radial growth lamellae for the beta-spherulites.