The secondary electron emission coefficient γ of a MgO protective layer with various crystallinities has been successfully measured by the γ-focused ion beam system with complete elimination of the charge accumulation problem by scanning-area adjustment techniques. It is found that the (111) surface has the highest γ from 0.14 to 0.26 in comparison with the other films with (200) and (220) crystallinities for operating Ne+ ions, while ranged from 0.03 to 0.24 for Ar+ ions, under operating ion energies from 50 eV to 500 eV throughout this experiment. These observations explain why the (111) crystallinity of the MgO protective layer plays an important role in lowering the firing voltages in AC plasma display panel compared to the films with other crystallinities.