Mechanical milling behavior of Mo Si Co powders has been investigated by using XRD, SEM and TEM techniques. The mixtures of elemental molybdenum, silicon and cobalt powders with a stoichiometry of Mo5 xCoxSi3 (x=0.5,1,2) were milled in a planetary mill up to 100 hours. It was found that three kinds of powder mixtures showed the same trend in the process of mechanical milling. First, Mo (Co, Si) supersaturated solid solution was formed (Moss). High energy milling led to a remarkable expansion of the solubility of Co, Si in molybdenum. The transformation of Moss to an amorphous phase was identified after a longer time milling. In the milling process, the grain size of Mo (Co, Si) decreased gradually and the internal stress increased sharply. After 40 hour milling, the grain size was reduced to about 8 nm. The SEM analysis of milled powders showed that the particle size increased initially with milling time. After 100 hours milling, particles exhibited a spherical morphology and the particle size was reduced to about 100nm.