The effect of the raw materials composition of Mo–Si powders mixture on mechanical alloying has been investigated using a high-energy vibratory type ball-milling machine. The phase transformation of powders mixture is characterized by means of X-ray diffractometer. The results show that the molybdenum silicide is formed by the mechanically induced self-propagating reaction in relatively broad composition range from the molar ratio of Mo:1.5Si to Mo:2.5Si. As the content of silicon increases, the amount of critical stored energy for mechanically induced self-propagating reaction is increased, whereas the favorable Mo–Si reactive interface is decreased, and the difficulty of formation of Mo(Si) supersaturated solid solution is enhanced. These factors prolong the incubation period of mechanically induced self-propagating reaction from 60 to 130 min. Compared to stoichiometry of MoSi2, the excess amount of Mo acts as accelerant, and the redundant Si acts as diluting agent.