Abstract: The tribochemical effects of Mo/2Si powder mixture in ball milling have been studied by X ray diffraction. It has been found that the mechanical milling process of the powder mixture was characterized by a decrease in the crystallite size and increase in the microstrain and effect temperature factor. The variation of the crystallite size,the microstrain, and the effective temperature factor took different pace at different milling duration. Namely, the crystallite size decreases largely in the early stage of milling, then it comes to stable stage in a range of milling duration and decreases gradually again at an extended milling duration. Finally the crystallite size keeps constant at a long enough milling duration. Contrary to the above, the microstrain increases considerably in the early stage of milling and comes to a nearly constant value at an extended milling duration, and the effective temperature factor comes to a limit value. Moreover, the changing speed of the different tribochemical effect factors is also different, so is that for the different phases comprising the particulate mixture. Namely, the brittle silicon particulates were easier to fracture than the ductile molybdenum particulates. Therefore, Si shows smaller microstrain and limit value of effect temperature factor than molybdenum. However, almost no tribochemical change was detected in the milling process of Mo/2Si powder mixture.