Alloy　catalysts　often　show　superior　effectiveness　in　the　growth　of　carbon　nanotubes/nanofibers　(CNTs/CNFs)　as　compared　to　monometallic　catalysts.　However,　due　to　the　lack　of　an　understanding　of　the　active　state　and　active　structure,　the　origin　of　the　superior　performance　of　alloy　catalysts　is　unknown.　In　this　work,　we　report　an　in　situ　transmission　electron　microscopy　(TEM)　study　of　the　CNF　growth　enabled　by　one　of　the　most　active　known　alloy　catalysts,　i.e.,　Ni-Co,　providing　insights　into　the　active　state　and　the　interaction　between　Ni　and　Co　in　the　working　catalyst.　We　reveal　that　the　functioning　catalyst　is　highly　dynamic,　undergoing　constant　reshaping　and　periodic　elongation/contraction.　Atomic-scale　imaging　combined　with　in　situ　electron　energy-loss　spectroscopy　further　identifies　the　active　structure　as　a　Ni-Co　metallic　alloy　(face-centered　cubic,　FCC).　Aided　by　the　molecular　dynamics　simulation　and　density　functional　theory　calculations,　we　rationalize　the　dynamic　behavior　of　the　catalyst　and　the　growth　mechanism　of　CNFs　and　provide　insight　into　the　origin　of　the　superior　performance　of　the　Ni-Co　alloy　catalyst.　©