Abstract Nanoindentation was performed in the (0 0 1) and (1 1 0) planes of magnesium oxide single crystal. The results showed that pop-in events were observed in both cases, which were accompanied by acoustic emissions. The energy released in a pop-in event was related to the acoustic emission energy. The pop-in indicated the onset of plastic deformation. Using the Hertzian contact theory and the measured pop-in load, the critical shear stresses for slipping on the (0 0 1) and (1 1 0) planes were estimated. The stress value for the (0 0 1) plane was about 17% higher than that of the (1 1 0) plane.