The changes in secondary electron emission coefficient(${gamma}$) and work function($Phi$$_{omega}$) have been studied on the surface of MgO protective layer aster plasma(Ar. $O_2$) treatment using ${gamma}$-focused ion beam (${gamma}$-FIB) system. The values of ${gamma}$ varied as follows: $O_2$-treated MgO > Ar-treated MgO > Non-treated MgO, and the work functions varied in the reverse order. The result indicates that both the physical etching and the chemical reaction of $O_2$-plasma removed the contaminating materials from the surface of MgO.