Carbon and oxygen on the Mo(l00) and Mo(l1l) surfaces can not be removed by bombarding the sample with argon ions at the ambient temperature. While carbon and oxygen can be reduced by bombarding the sample using nitrogen ions with primary energy of 1 keV and beam current of 6 μA for 10-15 min. This may be due to the adsorption of nitrogen reached saturation. After heating sample to 400-600℃ carbon and oxygen disappeared; at 600-650℃, Auger peak of nitrogen dropped suddenly and then decreased gradually with increasing temperature, carbon segregated simultaneously on the surface. Complete desorption of nitrogen took place at 730℃ and 775℃, A p(2×2)-C and a c(6×2)-C LEED patterns were observed respectively. After nitrogen desorbtion, the sample is cooled to room temperature, oxygen segregated on Mo(lll) surface. Repeated heating to above 650℃, carbon disappeared from the surface, nitrogen and oxygen appeared on the surface, until at 800℃ nitrogen and oxygen disappeared completely and a clean surface Mo(lll) LEED pattern was obtained. Then cooling the sample to room temperature, oxygen segregated on surface again. After bombarding the Mo(lll) surface with argon ions, carbon and nitrogen agan appeared on the surface. Heating to 700℃ and 760℃ oxygen and nitrogen disappeared and a p(6×6)-C LEED pattern was obtained.