When an amorphous polymeric substance is stretched in one direction the crystallization takes place and the fiber structure appears. The elasticity of fiber structure is characteristic in the coexistence of entropy elasticity with energy elasticity. It is proved thermodynamically that, if the energy elasticity if supposed to be based on the energy increase of crystallized past only we encounter the contradiction. Accordingly we must consider that the energy elasticity of fiber structure is not only induced by the deformation of crystallites but also by the deformation of segments in amorphous part. At this standpoint the degree of crystallization and the retractive force against the elongation is discussed.
