The changes in the work $function({Phi}_w)$ in the MgO protective layers after $plasma(Ar,;H_2)$ treatment have been studied using ${Upsilon}-focused$ ion beam $({Upsilon}-FIB)$ system. The ${Phi}_w$ was determined as follows: Ar-plasma $treatment({Phi}_w=4.52eV)$, $H_2-plasma$ $treatment({Phi}_w=5.65eV)$, and non-plasma $treatment({Phi}_w=4.64eV)$. The results indicated that the H-plasma could not make any effective physical etching due to the small masses of hydrogen atoms and molecules while the hydration of H-plasma could grow some contaminating materials on the surface of MgO.?