Mechanochemical reaction by mechanical alloying (MA) was investigated for powder mixtures of Ni-33mol%Al. Exothermic reaction was striking in the MA powders of fine layered structures observed in an intermediate stage of the MA process, and the MA powders induced combustion synthesis in air (room temperature) and vacuum heating at a relatively low temperature (near 673K). The experimental results were discussed on the basis of the model created by Bhattacharya and Arzt. The reason for the activation of combustion synthesis by MA is considered to be extra diffusivity caused by ultrahigh dislocation density, such that the high reaction heat forms a liquid phase locally which induces combustion synthesis