Magnesium silicide (Mg2Si) is a promising as an environmental friendly thermoelectric material. It is difficult to synthesize high-purity Mg2Si because of the boiling of Mg and the oxidation of Mg. In order to avoid the disagreement of stoichiometric ratio and the oxidation, we have performed high-pressure synthesis. And also the high-pressure X-ray diffraction by the synchrotron radiation source was carried out to find synthetic condition under pressure, and we have found that Mg2Si was synthesized at 1.0 GPa beyond 573 K. Because high synthetic temperature causes the oxidation of Mg, the high-pressure synthesis by means of a piston cylinder apparatus performed at 673 K. The synthesized compounds were evaluated by XRD analysis and thermoelectric measurement. Because the size of compounds synthesized under pressure is small, we also developed and evaluated the new equipment of the thermoelectric measurement for such as small sample. (c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim