Various nano-fillers can introduce specific functions into polymer and expand their application areas. Myriad properties, such as mechanical, electrical, thermal, or magnetic properties can be combined with original polymer characteristics, including flexible, light weight, and ease of use. These composites can be used to produce multi-functional fibers as the next generation textile or fabrics. In this research, Polyacrylonitrile (PAN) is adopted as the main polymer with different nano-fillers, such as carbon nanotube (CNT), iron oxide nanoparticle, and graphene oxide nanoribbon (GONR). Using gel-spinning technology, PAN-based composite fibers are fabricated in single- or bi-component fibers. Fibers are also characterized for their structure, morphology, mechanical properties, as well as for their electrical, thermal, or magnetic properties. For example, bi-component fibers with polymer sheath and polymer-CNT core as well as polymer-CNT sheath and polymer core are processed. With electrical and thermal conductivity introduced by CNT, such bi-components fibers can be applied for wearable electronics or for thermal management. Joule-heating effect owing to applied electrical current on single component PAN/CNT fibers is also investigated. With controllable electrical conductivity and fiber temperature, this active functional fiber can be applied for temperature regulation fibers or new carbon fiber manufacturing process. Another example is magnetic fiber with superparamagnetic iron oxide nano-particles. These novel magnetic fibers with high strength can be used for actuator, inductors, EMI shielding, or microwave absorption. GONR is also discussed and used to reinforce PAN-based fibers. Several theoretical models are considered to analyze the observed results.