Mo powders were mechanically ground in a carbon steel, a stainless steel and a tungsten carbide vial with several sample to ball weight ratios yielding the nano-size crystallites.The time dependences of the grain size, the compositional change and the distortion accompanied by the ball milling were investigated for the ball-milled powders. Nano-size Mo alloys with an average diameter of 6nm were obtained by the ball milling, regardless of the kind of the vial material. Large compositional changes of Fe, Fe+Cr+Ni and W content were involved by the grain size refinement after the long time milling in the carbon steel, the stainless steel and the tungsten carbide vial, respectively. The decrease of the weight ratio of sample to ball was effective for the preparation of a specified grain sized powders within the decreased milling time, though it was accompanied by a large distortion.