M23X6 (M-transition metal, X-nonmetal) phases widely exist in alloy steels and some other multicomponent alloys, but were seldom solidified from binary alloys. In the present work, the alloy melt with the nominal composition of Co23B6 phase, Co79.3B20.7, was undercooled to different degrees, and metastable Co23B6 phase was solidified at undercooling beyond 60 K owing to its lower energy barrier to nucleation than the equilibrium Co3B phase. The Co23B6 phase formed at high temperature can be retained to room temperature if the cooling rate is larger than 25 K min(-1). This phase was confirmed to be the classic Cr23C6-type structure (space group ) with a lattice parameter, measured by Rietveld refinement method, of a = 10.4912(1) .