(Received January 10, 1966)
Kinking of fibrils in oriented polyethylene under stress was investigated and three kinds of kink band were found. They were studied with an optical and an electron microscope and by means of X-ray diffraction. From the relationship between the directions of the c-axis, the applied forces and the kink bands, two types of kinking are proposed. One is by crystalline slip along molecular chains accompanied by rotation of the c-axis and the other is by a mechanism similar to twinning induced by shear stress along the king boundary. The kink band formed by compression of an oriented plate along its fiber axis was always oriented in such a way that it appeared to be normal to the fiber axis in the flat surface of the plate and oblique to it in the lateral face. Thus it is suggested that grain boundaries play an important part in the formation of the kink band.