As　one　of　the　most　remarkable　oxygen　evolution　reaction　(OER)　electrocatalysts,　metal　chalcogenides　have　been　intensively　reported　during　the　past　few　decades　because　of　their　high　OER　activities.　It　has　been　reported　that　electron-chemical　conversion　of　metal　OER　chalcogenides　into　oxides/hydroxides　would　take　place　after　the　OER.　However,　the　transition　mechanism　of　such　unstable　structures,　as　well　as　the　real　active　sites　and　catalytic　activity　during　the　OER　for　these　electrocatalysts,　has　not　been　understood　yet;　therefore　a　direct　observation　for　the　electrocatalytic　water　oxidation　process,　especially　at　nano　or　even　angstrom　scale,　is　urgently　needed.　In　this　research,　by　employing　advanced　Cs-corrected　transmission　electron　microscopy　(TEM),　a　step　by　step　oxidational　evolution　of　amorphous　electrocatalyst　CoSx　into　crystallized　CoOOH　in　the　OER　has　been　in　situ　captured:　irreversible　conversion　of　CoSx　to　crystallized　CoOOH　is　initiated　on　the　surface　of　the　electrocatalysts　with　a　morphology　change　via　Co(OH)(2)　intermediate　during　the　OER　measurement,　where　CoOOH　is　confirmed　as　the　real　active　species.　Besides,　this　transition　process　has　also　been　confirmed　by　multiple　applications　of　X-ray　photoelectron　spectroscopy　(XPS),　in　situ　Fourier-transform　infrared　spectroscopy　(FTIR),　and　other　ex　situ　technologies.　Moreover,　on　the　basis　of　this　discovery,　a　high-efficiency　electrocatalyst　of　a　nitrogen-doped　graphene　foam　(NGF)　coated　by　CoSx　has　been　explored　through　a　thorough　structure　transformation　of　CoOOH.　We　believe　this　in　situ　and　in-depth　observation　of　structural　evolution　in　the　OER　measurement　can　provide　insights　into　the　fundamental　understanding　of　the　mechanism　for　the　OER　catalysts,　thus　enabling　the　more　rational　design　of　low-cost　and　high-efficient　electrocatalysts　for　water　splitting.