II-VI compound semiconductor ZnO has a potential for high radiation hardness since large threshold displacement energy of constituent atoms can be expected due to the small unit-cell volume and large bandgap energy of 3.37 eV. In order to study the radiation hardness, singlecrystalline c-axis-oriented O-polar ZnO films with and without two-dimensional electron gas, a Zn-polar ZnO bulk crystal, and a Ga-polar GaN bulk crystal for comparison, were irradiated by an 8 MeV proton beam using a tandem-type accelerator. The radiation damage increased the electrical resistance and decreased the photoluminescence (PL) intensity of these samples with the increase of proton fluence over specific threshold values. In agreement with the expectation, ZnO samples were revealed to have superior radiation hardness; the threshold fluences for the deterioration of PL intensity were 3×1013 p/cm2 for the GaN bulk crystal, 2×1014 p/cm2 for the ZnO bulk crystal, and 5×1014 p/cm2 for the two ZnO films, in accordance with the order of the threshold fluences for the electrical resistance increase. The effect of post-irradiation annealing was also studied for these damaged bulk crystals; both electrical and optical properties of the ZnO bulk crystal were almost recovered to the pre-irradiation values, however, only the electrical properties of the GaN bulk crystal were recovered, by the annealing up to 700°C. Such a rapid recovery of the ZnO bulk crystal indicates the easy annihilation of Zn vacancy complexes acting as non-radiative centers by the recombination with interstitial Zn atoms. Since the migration barrier height energy of interstitial Zn atoms is known to be so small that it might occur even at room temperature, we ascribed the superior radiation hardness of ZnO crystals to the restoration of damage-induced defects by a self-annealing effect during irradiation.