Abstract: Co-injection molded isotactic polypropylene specimens were made at three different core injection rates of 20, 40,60 mm/s. The microstructure of the molded specimens was observed by polarizing optical microscopy. On the basis of the numerical and experimental results, the temperature field, shear rate field, and shear stress filed at the selected locations were calculated and a formation criteria equation for cylindrulite was established in terms of energy variation. Most of all, a quantitative criterion was established for the formation of polypropylene cylindrulite during the processing, which suggested that the cylindrulite could be formed in the range of ［170, 222］ and ［252, 458］.