Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer whose fibrils can be easily formed by stretching because of its low tensile activation energy. We prepared two series of PTFE samples with different elongations and degrees of crystallinity, respectively. The morphology analysis of the first series determined that cavities were formed in the PTFE matrix under stretching, and this was a prerequisite for forming fibrils. The barrier between cavities became long and slim under tension, and grew into fibrils finally. It could be found that higher crystallinity is more suitable for forming fibrils. The main part of the fibrils' structure was semi-crystalline, and the degree of crystallinity could grow under stretching. Besides, the thickness of the lamellae decreased, while the crystalline grain refined at the same time. In the measurement of X-ray diffraction, it could be found that the lattice plane (108) was more sensitive to stretching, which could rotate to other positions. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3710-3717, 2013