General mode of deformation and the morphology of fracture surface in polymers have been studied systematically from the point of view of fluidity of molecular chains.Particularly for polystyrene, the mechanism of the propagation of fracture has been investigated in the light of morphology observed by the scanning type electron microscope.Materials may be classified roughly into two groups, ductile one and brittle one, in terms of the fluidity. The ductility and the brittleness depend on the temperature and the speed of fracture.In polymers, microfracture is first induced by uneven stress or defects in the material and then. Macrofracture occurs as the progress of the microfractures. Generally, crazing appears as the microfracture. In the case of ductile polymers, the craze grows into a necking, while in brittle polymer, e. g., polystyrene, the craze turns to a crack. The craze-induced fracture is a sort of ductile fracture and occurs along the boundary of the brittle and the ductile region giving rise to the so called mirror region. The ductile, mirror and brittle region present fibrous, granular and cleavage fracture, respectively.In polystyrene, namely in the case of low fracture speed, the crack propagates within the crazed region. The mirror region offers many fine structures, characteristic of the condition of deformation. In the case of low fracture speed, the mirror region presents a fibrous fracture caused by the breakdown of molecular chains (in the middle of the craze). As the fracture speed increases, the crack tends to propagate along the interface between the craze and the undeformed matrix, giving rise to the separation of molecular chains. The mirror region has a granular fracture. When the fracture speed is very high, the fast-moving crack propagates in the interface. A large tensile stresses generated at the tip of the crack tends to relax momentarily and, accordingly, the speed of the cracking slows down, as the crack shifts to another interface, leaving pieces of the broken craze on the matrix. Thus the crack path traverses between two craze-matrix interfaces. The fragment takes various kinds of shapes and striations, depending on the speed of crack propagation. As the crack propagation speed becomes even larger, the crack traverses between two interfaces of different crazes. In this case, fracture surface shows a little larger shapes and the striations.The results mentioned above are concerned with the shape of the mirror region or the mechanism of crack propagation in the craze. The mirror region seems to have a smooth surface by macroscopical observations. Microscopically the surface is generally granular, but the mirror near ductile region a fibrous fracture surface and the near brittle region, a cleavage surface.