Coarse-grained crystals of MgSi 2 with the C1 structure, produced by melting and slow cooling of constituent elements in a sealed molybdenum ampoule, have been compressed in vacuum at high temperatures. The crystals were brittle below 500 K and underwent plastic deformation above 550 K; the yield strength decreased rapidly with increasing temperature down to 10 MN/m 2 above 900 K. Transmission electron microscopy revealed that the plastic deformation was carried by glide of dislocations with the Burgers vector of ½<1 1 0>. It is concluded, based on the thermal-activation analysis of plastic deformation, that the deformation is controlled by the Peierls mechanism.