Plastic deformation of semi-crystalline polymer is a complicated process containing two processes: cavitation and lamellae fragmentation-reconstruction. The relationship between macroscopic yielding point and the two processes and the relationship between the two processes themselves have long been a hot topic for the past decades. In this study, the structural evolution of isotactic polypropylene (iPP) during uniaxial drawing is monitored by synchrotron small angle X-ray scattering (SAXS). The results suggest that before the yielding point, lamellae rotation could be observed because of the elastic deformation of the amorphous phase. On the contrary, lamellae tilting induced by crystallographic slip could be found only after the yielding point. And melting-recrystallization of the lamellae takes place afterward. Cavities can be triggered before or after the yielding point. The cavities are oriented mainly perpendicular to the loading direction. The development of nano-cavities into micro-cavities before the yielding point will lead to the fracture of the specimen. Otherwise, a stable neck could be formed if the nano-cracks coalesce into micro-cavities after the yielding point. The micro-cavities are oriented along loading direction. Generally, annealing shows no influence on the lamellae melting-recrystallization. The thickness of the newly formed lamellae is in linear proportion to the reciprocal of the drawing temperature, irrespective of the original lamellae thickness. However, annealing could greatly promote the formation of the cavities because of the enhanced connection between the amorphous phase and the lamellae.