Metastable mesophases can strongly influence the physical properties of polymer fibers. It is therefore of high importance to study the structure, formation and behavior of such phases. Structural analysis can be used to tailor fiber properties, optimize manufacturing processes and ensure quality control. Wide-angle X-ray scattering (WAXS) is the ideal technique to study the nanostructure of polymer fibers. A monochromatic X-ray beam hits a fiber and its nanostructure scatters part of the beam elastically. The measured scattering pattern is like a fingerprint of the fiber structure and yields information about crystallinity, crystal sizes, crystal orientations, distances between lattice planes and can also provide information about highly oriented non-crystalline mesophases. We present several examples about the structural analysis of polymer fibers with WAXS. In particular, we focus on monocomponent and on bicomponent fibers with core-sheath geometry, which show highly oriented non-crystalline mesophases. Specifically, we will discuss physical properties and WAXS results from biodegradable polymer fibers of poly-3-hydroxybutyrate (P3HB) and also results from polymer optical fibers (POFs) of cyclo-olefin (COP) and tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride terpolymer (THV).