The behavior of Mg2Si synthesis in solid-state has been evaluated in employing the elemental mixture of raw powder, Mg and Si powder. In particular, the influence of the refinement of Si particles by the plastic working on in-situ formation of Mg2Si is discussed based on the thermal analysis results. In refining Si particles via the repeated plastic working (RPW) on the elemental mixture of Mg and Si powder, the solid-state synthesis of Mg2Si starts at lower temperature, for example at about 423K, compared to the mixture raw powder with that at 773K. The additional plastic working on the mixture causes the in-situ synthesis of Mg2Si intermetallics, which is similar to the behavior in the conventional mechanical alloying. TEM observation indicates that the particle size of Mg2Si and non-reacted silicon is 30-80nm and 500nm-1 mu m, respectively. The starting temperature of the Mg2Si synthesis, however, shifts to a higher temperature again, when a lot of Mg2Si intermetallics were synthesized by the further plastic working. This is because the interdiffusion of magnesium and silicon becomes sluggish due to these species that must go through the increase amount of the compounds to react. That is, in-situ synthesized fine Mg2Si exists as a barrier to obstruct the reaction between magnesium and silicon particles.