The stress-strain behavior of mainly β-form of isotactic polypropylene was measured at various temperatures and compared with those of α-form of the same grade sample. The results show that β-form sample has a lower yield strength and a higher impact strength than that of the α-form. The change of crystalline structure during stretching was studied by X-ray differaction technique. The difference in mechanical behavior of α- and β-forms was explained qualitatively with molecular model of deformation and structure transformation during strecting.