Motivated by recent impressive works on high catalytic reactivities of nano-scaled gold particles on oxides, we have investigated the atomic and electronic structures of gold nano-particles adsorbed on MgO(001) by first-principles calculations based on the density functional theory. We show the energetics of various adsorbed structures of Au_n (n = 1∼4 and 8) on MgO(001) with or without oxygen vacancies and discuss the effect of vacancy on the stability of nano-particles on the surface.