A quatitative and quantitative analysis of the microtexture/ microstructure in several experimental boron nitride fibers was investigated by X-ray diffraction and electron microscopy according to their mechanical properties. Young's modulus was controlled by grain orientation. The disordered microstructure of low modulus fibers consisted of amorphous regions and nanometer-sized crystallites arranged in a wide range of orientation. In contrast, high modulus samples showed a coarse-grained microtexture in which crystallites were ordered in a mesohexagonal arrangement along the fiber-axis. Cracks were easily propagated as crystallinity increased, leading to decreased failure strain. The strength-microstructure relationship was obscured by the fiber porosity and defects.