An increase of ZnO crystal quality after irradiation by Nd:YAG laser till intensity 3.2 MW/cm2, the first threshold, was observed. The facts is explained by decrease of donor-acceptor pairs' concentration which manifests as decrease of 2.3 eV band intensity and simultaneously increase of 3.27 eV exciton band intensity in photoluminescence spectrum. Further increase of the laser radiation up to 212.0 MW/cm2, the second threshold, leads to appearance of extended defects - cracks - on the irradiated surface of crystal. This effect is explained by mechanical compressive stress arising on the irradiated surface of the crystal due to non-homogeneous heating of the crystal by strong absorption of the laser radiation. Increase of the laser radiation up to 290.0 MW/cm2, the third threshold, leads to "black ZnO" formation on the irradiated area of the crystal. Investigation of the irradiated crystal surface morphology and its resistivity by atomic force microscope showed that morphology was not changed and resistivity decreased by 10^3 times due to increase of concentration of Zn interstitial atoms. Investigation of the nature of "black ZnO" using scanning electron microscope showed formation of nano particles with diameter up to 20 nm in the irradiated area of the crystal which are formed due to aggregation of Zn interstitial atoms.