Samples of isotactic polypropylene(PP) were subjected to stress-relaxation experiments after simple tensile tests at four strain rates and at different levels of temperature. The relaxation moduli were determined in the range of temperature between 20 and 80 degrees C with a relaxation period of 1200s duration. The activation energy value of the shift factor was determined using the time-temperature superposition principle. The calculated stress-strain curves and stress-relaxation curves were obtained from constitutive equations based on an overstress theory in which the temperature dependence of viscosity and the activation energy were considered. The temperature dependence of viscosity was amenable to an Arrhenius type equation. The quasi-equilibrium stress depends on both the current strain and the temperature. The calculated results were obtained by the proposed constitutive equation and compared with the data. The proposed constitutive equations based on the overstress model explain well the viscoelastic-plastic behavior of PP samples.