The structure of pyrolitic boron nitride (BN) samples assembled to a package and irradiated with protons has been studied with Raman spectroscopy and X-ray diffraction. Shift and change of shape of X-ray bands and change of Raman band shape have been found to correlate with the calculated values of energy losses along the proton track. Ionizing constituent of irradiation is responsible for a change of the state of grain boundaries and partial recrystallization of pyrolitic BN. Displacive constituent of irradiation is responsible for stacking fault formation in the grains. A mechanism of the effect of ionizing constituent of irradiation has been introduced, where energy release on grain boundaries and break of chemical bonds on the boundaries result in partial recrystallization of the material. According to this mechanism the recrystallization is much more feasible in pre-irradiated materials.