Polymer materials are often used after blending or compounding, which is an efficient route to combine useful properties of each polymer component to obtain high-performance and low-cost products. However, most polymer blends are immiscible. Moreover, performance of multiphase polymer materials is determined not only by properties of polymer components, but also by the multiphase structure. Especially, properties of blends can be significantly improved by control over phase structures during processing, which is essential for preparing high-performance polymer materials. Polylactide (PLA), a biodegradable material with high strength and modulus, has attracted extensive attention. We mainly investigated in situ formation of nanofibrils in PLA blends by elongational flow field to enhance the performance. The main research involves: (1) Toughening PLA without losing its strength was realized by introducing poly(butylene succinate) (PBS) nanofibrils via under elongational flow field. Meanwhile, PBS nanofibrils induced the highly oriented PLA kebabs, displaying hybrid shish-kebab structures. The good interfacial interactions between the PBS nanofibrils and PLA matrix lead to simultaneous strengthening and toughening of PLA. (2) The nanosheet structures evolved from PBS nanofibrils show an outstanding ability to not only achieve the simultaneous strengthening and toughening of PLA, but also to improve gas barrier properties of PLA film. (3) Various biopolymers were in PLA to enhance the ductility. We created in situ nanofibrils of poly(butylene adipate-co-terephthalate) (PBAT) with better ductility and lower Tg in PLA. Due to its large specific surface area and its partial miscibility with PLA, PBAT nanofibrils synergistically promoted the formation of oriented crystals of PLA under flow field, which significantly improved the comprehensive performance of blends. (4) Highly oriented PLA crystals induced by flow field showed high mechanical and barrier properties, which we