The　extremely　high　structural　tolerance　of　ceria　to　oxygen　vacancies　(Ov)　has　made　it　a　desirable　catalytic　material　for　the　hydrocarbon　oxidation　to　chemicals　and　pharmaceuticals　and　the　reduction　of　gaseous　pollutants.　It　is　proposed　that　the　formation　and　diffusion　of　Ov　originate　from　its　outstanding　reduction　property.　However,　the　formation　and　diffusion　process　of　Ov　over　the　surface　of　ceria　at　the　atomic　level　is　still　unknown.　Herein,　the　structural　and　valence　evolution　of　CeO2　(111)　surfaces　in　reductive,　oxidative　and　vacuum　environments　from　room　temperature　up　to　700　°C　was　studied　with　in　situ　aberration-corrected　environmental　transmission　electron　microscopy　(ETEM)　experiments.　Ov　is　found　to　form　under　a　high　vacuum　at　elevated　temperatures;　however,　the　surface　can　recover　to　the　initial　state　through　the　adsorption　of　oxygen　atoms　in　an　oxygen-contained　environment.　Furthermore,　in　hydrogen　environment,　the　step-CeO2　(111)　surface　is　not　stable　at　elevated　temperatures;　thus,　the　steps　tend　to　be　eliminated　with　increasing　temperature.　Combined　with　first-principles　density　function　calculations　(DFT),　it　is　proposed　that　O-terminated　surfaces　would　develop　in　a　hypoxic　environment　due　to　the　dynamic　diffusion　of　Ov　from　the　outer　surface　to　the　subsurface.　Furthermore,　in　a　reductive　environment,　H2　facilitates　the　formation　and　diffusion　of　Ov　while　Ce-terminated　surfaces　develope.　These　results　reveal　dynamic　atomic-scale　interplay　between　the　nanoceria　surface　and　gas,　thereby　providing　fundamental　insights　into　the　Ov-dependent　reaction　of　nano-CeO2　during　catalytic　processes.　©　2023　Chinese　Society　of　Rare　Earths