The ultra fast synthesis of nanocrystalline Mg2Si was carried out using microwave radiation. The elemental precursors were first milled together under dry conditions to get fine particles. The resulting mixture of powders of Mg and Si was cold pressed before being heated by microwave irradiation. Precursors and products were analyzed by X-ray diffraction and scanning electron microscopy. The high energy ball milling parameters utilized to prepare the reactive powders have quite an influence on the behavior of the mixture under irradiation. Moreover, SEM imaging demonstrates that the power and time of irradiation are crucial for the grain growth of the Mg2Si and must be adequately controlled in order to avoid the decomposition of the phase. Our results show that we successfully managed to easily and quickly synthesize homogeneous nanocrystalline Mg2Si with particle size smaller than 100 nm using a microwave power of only 175 W for two minutes on powders ball milled for two hours.