After bombarding the Mo (111) surface with nitrogen ions of the energy of 1 keV and the beam current of 6 μA for 15 min, a very strong Auguer peak of nitrogen appeared in AES. The LEED observation showed that surface structure was disordered after annealing in the temperature range from room temperature to 450℃. The stable square pattern appeared at 562℃ can be interpreted in terms of c(3×1)-N, O overlayer structure on the Mo (433) faceting planes.