Using the specially designed mechanochemical ball-mill equipment,ultramicro molybdenum nitride powders were prepared from pure mo- lybdenum powders in ammonia atmosphere at room temperature by high-energy ball milling.The structure and the particle size of the pow- ders were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),and transmission electron microscopy(TEM).The results show that the mass ratio of grinding media to powder was 8:1,after milling for 30 h the Mo_2N of fcc structure was obtained,and the average particle size of the powders was around 100 nm.It is found that the chemisorption of ammonia onto the fresh molybdenum surfaces created by milling was the predominant process during solid-gas reaction,and the energy input due to introduction of highly dense grain boundaries and lattice defects offered the activation energy for the transition from Mo-N chemisorption to molybdenum nitride.In addition, the change of Mo electronic undersaturation induced by the grain refining accelerated the bonding between Mo and N.The mechanism model of whole nitriding reaction was given.During the high-energy ball milling processing,the rotational speed of milling played a critical role in determining the overall reaction speed. Using the specifically designed mechanochemical ball-mill equipment, ultramicro molybdenum nitride powders were prepared from pure mo- lybdenum powders in ammonia atmosphere at room temperature by high-energy ball milling. The structure and the particle size of the pow-ders were investigated by X -ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that mass ratio of grinding media to powder was 8: 1, after milling for 30 h the Mo_2N of fcc structure was obtained, and the average particle size of the powders was around 100 nm. It is found that the chemisorption of ammonia onto the fresh molybdenum surfaces created by milling was the predominant process during solid-gas reaction, and the energy input due to introduction of highly dense grain boundaries and lattice defects offered the activation energy for the transition from Mo-N chemisorption to molybdenum nitride. In addition, the change of Mo electronic undersaturation induced by the grain refining accelerated the bonding between Mo and N. the mechanism model of whole nitriding reaction was given. During the high-energy ball milling processing, the rotational speed of milling played a critical role in determining the overall reaction speed.