beta phase isotactic polypropylene (beta-iPP) with the beta phase lamellae oriented parallel to the melt extrusion direction, was employed to investigate the deformation-induced structure evolution at various temperatures (25, 80, 110, 130 and 140 degrees C). The orientation change of beta phase during deformation greatly influences the beta to alpha phase transformation and its temperature dependence. At temperatures lower than 110 degrees C, the orientation of beta phase is almost unchanged during deformation, and void or crack forms before fragmentation and reorientation of beta phase. The crystal size change of beta phase is small, and the defolding of the beta lamellae triggers the beta to alpha phase transformation. As the deformation temperature rises to 130 and 140 degrees C, the reorientation of beta phase occurs gradually upon stretching, and the size of micro-voids decreases due to the fact that less beta crystal fragmentation takes place at high draw temperature than that at low temperature. The beta to alpha phase transformation is mainly induced by intra-lamella slip, and the trend of crystal size change of beta phase is larger. The chains orientation changes from perpendicular to the stretching direction in beta phase to parallel to the stretching direction in alpha phase is achieved by the chains defolding of beta phase along the stretching direction at temperatures lower than 110 degrees C, and it is through the chains reorientation of beta phase along the stretching direction at temperatures of 130 and 140 degrees C. The crystal size of alpha phase of the deformed beta-iPP during deformation depends on the dynamic balance of the breakage of existing alpha crystal and the formation of new crystal through phase transformation. Specially, at draw temperature of 25 degrees C, the slippage of beta phase relieves the breakage of alpha phase crystal, which indicates that the high content of beta phase crystal really accounts for the toughening effect on iPP. (C) 2012 Elsevier Ltd. All rights reserved.