Understanding　the　high-temperature　initial　oxidation　mechanism　and　its　dynamic　processes　is　generally　important　because　a　small　amount　of　initial　oxidation　can　induce　rapid　corrosion　and　cause　catastrophic　failure.　We　reveal　the　mechanism　of　initial　oxidation　in　a　third-generation　Ni-based　superalloy　by　in　situ　visualizations　and　investigations　of　the　nano-　and　atomic-scale　dynamic　processes　from　room　temperature　to　900　degrees　C　with　a　Cs-corrected　environmental　transmission　electron　microscope.　The　initial　oxidation　starts　from　the　gamma/gamma＇　interface　at　low　temperatures.　The　high-temperature　oxidation　process　with　deficient　oxygen　prefers　oxidation　sites　at　the　cross-junctions　of　the　gamma/gamma＇　interfaces.　The　growth　rate　of　the　oxide　nanoparticles　depends　on　the　oxidation　temperatures,　with　a　low　rate　below　600　degrees　C　and　a　rapid-growth　speed　at　temperatures　higher　than　700　degrees　C.　Mass　transfer　from　the　gamma＇　and　gamma　phases　to　the　gamma/gamma＇　interface,　particularly　at　the　cross-junctions　of　these　interfaces,　is　observed　to　cause　oxide　accumulation.　This　study　provides　a　direct　observation　of　the　interphase,　interface-junction-initiated　outward　oxidization　including　Al　and　Cr　elements.　These　results　shed　light　on　the　initial　oxidation　mechanisms　of　materials　containing　a　second　phase　with　interphase　interfaces　and　the　materials　at　deficient　oxygen　conditions　as　well　as　those　with　protection　coating　layers.　(C)　2021　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.