A constitutive equation for cold-rolled polypropylene is presented based on a modified two-phase model for semicrystalline polymer, which combines the crystalline network that obeys the Langevin based theory and the entangled network with slip-linked character The microstructure of the rolled semicrystalline polymer consists of packed crystalline inclusions which are separated by adjacent amorphous regions. A theoretical description of the stress strain behavior is obtained by the sum of the stress effects from crystalline and entangled networks. The microstructural observations by scanning electron microscopy, X-ray pole figures and differential scanning calorimetry show that polymer chains are aligned along the rolling direction. The stress-strain equation can be obtained from the modified two-phase model and verified by experimental results.