Polymer composite has been under rapid development with advancements in polymer chemistry, synthetic fibers, and nanoparticles. With advantages such as lightweight, corrosion resistance, and tunable functionalities, polymer composite plays a significant role in various applications such as aerospace, wearable electronics, energy storage systems, robotics, biomedicine, and microelectronics. In general, polymer composite can be divided into particulate-filled, fiber-filled, or network-filled types depending on the manufacturing process and internal structure. Over the years, fabrication processes on the macro- and micro-scales have been extensively explored. For example, lamination, fiber tow steering, and fiber spinning correspond to meter, millimeter, and micrometer scales, respectively. With the development of nanoparticles and their exceptional material properties, polymer nanoparticle composite has shown promising material property enhancements. However, the lack of economical solutions to achieve nanoscale nanoparticle morphology control limits the reinforcement efficiency and industrial applications. This dissertation focuses on utilizing additive manufacturing as a tooling method to achieve nanoparticle morphology control in polymer nanocomposite fibers. Chapter 1 gives a thorough background review regarding fiber composite, additive manufacturing, and the importance of nanoparticle orientation. Two types of nozzle designs, concentrical and layer-by-layer, are 3D printed and combined with the dry-jet-wet fiber spinning method to create continuous fibers with internal structures. Chapters 2 to 5 correspond to four stages of my research, namely, (2) multi-material fiber spinning, (3) interfacial-assisted nanoparticle alignment, (4) microscale patterning, and (5) nanoscale patterning. The achieved feature resolution also improves from 100 µm, 10 µm, 2 µm, to 170 nm, respectively. The process-structural-property relationship of polymer nanocomposite fibers is also investigated with applications demonstrations including sensors, electrically conductive fibers, thermally conductive fibers, and mechanically reinforced fibers. At last, Chapter 6 gives a summary and some future perspectives regarding fiber composites.