In this study, the structural factors controlling the yield in isotactic polypropylene materials were theoretically investigated. To describe the yielding behavior of spherulitic polypropylenes, we introduced a new structural unit, lamellar clusters, which are several stacked lamellae bound by tie molecules. It was shown that tie molecules between adjacent lamellar clusters produce a concentrated load acting on the cluster surface, leading to the bending deformation of the lamellar dusters. The yielding behavior can be explained if one assumes that the disintegration of the lamellar clusters occurs when the elastic-strain energy stored by the bending deformation reaches a critical value. By applying the fracture theory of composites to a system consisting of lamellar clusters and tie molecules, we found the yield stress sigma(y) to be proportional to root 2E(Y)U(y), in which E-Y is the Young's modulus and U-y is the yield energy. The proportional coefficient between sigma(y) and root 2E(Y)U(y) depends only on the cluster size and tie-molecule density, so this proportionality is expected to be true for other spherulitic semicrystalline polymers such as polyethylenes, being independent of temperature and tensile rate.