Densely woven highly crystallized biocompatible sodium-potassium niobate Na0.35K0.65NbO3 fibers with an average diameter of 100-200 nm and several hundreds of microns in length were sintered by the sol-gel calcination-assisted electrospinning technique. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) confirmed preferential cube-on-cube [001] orientation of nanocrystals within the fiber's body, separated by a low angle grain boundary. The Williamson-Hall method was employed to analyze the broadening of XRD reflections and to accurately determine the size and intrinsic strain of nanocrystal fiber aggregates. The main objective of this article is to test the potential capacity of direct XRD analysis to noninvasively control crystallite size and lattice distortion in core-shell coaxial nanofibers.