Lightweight plastic materials are significant for saving resource and energy, reducing environmental pollutions, and achieving sustainable development. Foam injection molding is a promising technology to provide lightweight plastic components. However, it suffers from the component's poor mechanical properties and surface appearance. Herein, we reported a novel strategy to prepare lightweight, tough and surface defect-free polypropylene/polytetrafluoroethylene (PP/PTFE) nanocomposite parts by combining an in-situ fibrillation technology and a nanocellular injection molding technology. The nano-fibrillary PP/PTFE nanocomposite was first prepared through an in-situ way based on twin-screw compounding. The SEM, rheological and DSC analysis, combining with the online optical microscope observation, demonstrated the network-structure PTFE nano fibrils and their positive effects in improving melt strength and promoting crystallization. With the nano-fibrillary nanocomposite, we achieved nanocellular foaming for the first time using the foam injection molding process. The nanocellular PP/PTFE nanocomposite foam showed sharply enhanced mechanical properties than the regular PP foam, even much better than the unfoamed PP part in both strength and ductility. In particular, the nanocellular foam's impact strength was 700% higher than the regular foam, and even 200% higher than the unfoamed case. Moreover, unlike the regular foam, the nanocellular PP/PTFE nanocomposite foam showed amazing surface appearance without any silver or swirl marks. More importantly, the whole process was facile, flexible, efficient and easy to scale-up, and it can be easily extended to other materials. The remarkable mechanical performance and surface appearance, combined with the flexible and extendable process, endows the nanocellular PP/PTFE nanocomposite foam a promising future in many advanced applications where both lightweight and mechanical integrity are required.