Upon tensile stretching up to ultimate macroscopic failure, several structural transitions are involved in semicrystalline polymers. Here, we report experimental observations of such transitions such as fibrillation, cavitation and failure at lamellar and molecular scales. Despite the complicated semi-crystalline structures, all observed results could be very well understood by considering a structural model of interpenetrated networks of hard crystalline skeleton and soft entangled amorphous network in the system [1]. Fibrillation is proven to be a melting and recrystallization process due to highly stretched entangled amorphous network that generated enough stress over the critical destruction stress for crystalline blocks [2,3]. Only for samples possess low crystallinity the model of two-phase interpenetrated networks fails. In such case, one has to consider a three phase model including a third inter-lamellar stacks soft phase that deforms first which effectively delays the occurrence of fibrillation [4,5]. Cavitation at small deformations occurs due to the failure of strong crystalline skeleton [6,7]. Ultimate failure starts from a generation of micro-cavities in highly stretched fibrillar structure due to tie chain breaking followed by disentanglements [8]. Acknowledgement: This work is supported by the National Natural Science Foundation of China (21134006, 51525305). References: [1] Y. Men, J. Rieger, G. Strobl, Phys. Rev. Lett. 2003, 91, 095502. [2] Z. Jiang, L. Fu, Y. Sun, X. Li, Y. Men Macromolecules 2011, 44, 7065. [3] Y. Wang, Z. Jiang, Z. Wu, Y. Men Macromolecules 2013, 46, 518. [4] Y. Sun, L. Fu, Z. Wu, Y. Men Macromolecules 2013, 46, 971. [5] J. Zhao, Y. Sun, Y. Men Macromolecules 2016, 49, 609. [6] Y. Men, J. Rieger, J. Homeyer, Macromolecules 2004, 37, 9481. [7] Y. Wang, Z. Jiang, L. Fu, Y. Lu, Y Men, PLoS ONE, 2014, 9(5), e97234. [8] Y. Lu, Y. Wang, R. Chen, J. Zhao, Z. Jiang, Y. Men Macromolecules, 2015, 48, 5799.