Recently, developments on a field of polymer physics are remarkable both on the theoretical and experimental viewpoints. Though fracture behavior on high polymers is not yet clear at present time, some theoretical and experimental advances enable us to utilize for industrial purposes. So, in this treatise, theory of mechanical break down, influences of molecular structure on tensile strength are discussed. The tensile strength increases with an increase of specific cohesion, degree of stretching, crystallinity and cross-linking. Fractures proceed on interfaces of spherulites and depend on its size. The time-temperature superposition principle is available for fracture behavior on both amorphous and crystalline high polymers. From this principle we can decide high speed strength by measuring of low temperature. Except Bueche's theory, theoretical predict of fracture properties have failed.