Polymer nanofibers have highly configurable ability and good guiding properties, which is desirable for ultracompact microphotonic devices and miniaturized photonic integrated circuits. A polymer nanofiber-poly (trimethylene terephthalate) (PTT) nanofiber with good mechanical and optical properties is introduced, which is fabricated by one-step drawing process. As a subwavelengh waveguide, polymer nanofiber has tight-confinement ability and large evanescent optical field. A varies of PTT nanofibers-based ultracompact structures and devices (such as bends, rings, M×N optical coupling splitters, and Mach-Zehnder interferometers) are described in detail. These structures and devices have advantages of compact structure and low loss. Finally, the properties and application prospects of the polymer nanofibers and nanofiber devices are summarized.