We　report　the　unique　thermal　and　mechanical　stability　of　a　metastable　B2　phase　embedded　in　a　metallic　glass　matrix　at　liquid-nitrogen　temperature.　The　B2　phase　is　stabilized　by　the　constraints　of　the　hard　amorphous　matrix,　which　result　from　the　increasing　hydrostatic　pressure　during　cryogenic　cooling.　The　stability　of　the　B2　phase　contributes　to　the　high　strength　and　ductility　of　CuZr-based　bulk　metallic　glass　composites　at　cryogenic　temperature.　This　discovery　is　expected　to　provide　new　strategies　for　designing　strong　and　ductile　materials　containing　a　metastable　phase.　(C)　2015　Elsevier　B.V.　All　rights　reserved.