Phase　transformation　is　an　important　deformation　and　blunting　mechanism　at　the　crack　tip　of　body-centered　cubic　(BCC)　metals.　The　BCC-to-face-centered　orthogonal　(FCO)　transformation　was　only　predicted　by　ab　initio　calculations.　In　this　paper,　the　deformation　behavior　of　the　nano-sized　single-crystalline　Mo　is　in　situ　investigated　in　a　high-resolution　transmission　electron　microscope　at　room　temperature.　We　uncover　the　strain-induced　BCC-to-FCO　phase　transformation.　The　FCO　Mo　grows　in　a　layer-by-layer　fashion.　It　is　a　metastable　phase　after　stress　releasing,　as　confirmed　by　our　ab　initio　calculation.　The　BCC-to-FCO　phase　transformation　is　realized　via　lattice　shearing　and　shuffling.　IMPACT　STATEMENT　This　paper　provides　direct　in　situ　atomic-scale　evidence　of　BCC-FCO　phase　transformation,　which　has　never　reported　on　the　experimental　level.