After bombarding the Mo (001) and Mo (110) surfaces contained trace amount of carbon and oxygen with nitrogen ions of the energy of 1 keV and the beam current of 6 μA for 10-15 min, a very strong Auger pcak of nitrogen appeared in AES.The LBED observation showed that surface structure was disordered from room temperature to 350℃ annealing. When the sample was heated to the temperature range530-650℃, three types of LEED pattern, i. e.,c(2×2)-N, p(2×2)-N and (4(21/2)×21/2 R45°-N, O formed on Mo(001). In the case of Mo (110) only one type LEED pattern of c(7×3)-N was observed.The fact that the LEED structure of surface related to thermal desorption indicates that ordered structure depends on coverage of notrogen and temperature, and provi--des some information about phase transformation on the surface.