Understanding　the　mechanism　of　the　formation　and　rupture　of　conductive　filaments　in　HfO2-based　memristors　is　essential　to　solve　the　problem　of　scalability　of　the　devices.　Here,　Zhang　et　al.　visualize　this　process　by　tracking　atomic-scale　evolution　of　conductive　filaments　during　resistive　switching　cycles.　The　resistive　switching　effect　in　memristors　typically　stems　from　the　formation　and　rupture　of　localized　conductive　filament　paths,　and　HfO2　has　been　accepted　as　one　of　the　most　promising　resistive　switching　materials.　However,　the　dynamic　changes　in　the　resistive　switching　process,　including　the　composition　and　structure　of　conductive　filaments,　and　especially　the　evolution　of　conductive　filament　surroundings,　remain　controversial　in　HfO2-based　memristors.　Here,　the　conductive　filament　system　in　the　amorphous　HfO2-based　memristors　with　various　top　electrodes　is　revealed　to　be　with　a　quasi-core-shell　structure　consisting　of　metallic　hexagonal-Hf6O　and　its　crystalline　surroundings　(monoclinic　or　tetragonal　HfOx).　The　phase　of　the　HfOx　shell　varies　with　the　oxygen　reservation　capability　of　the　top　electrode.　According　to　extensive　high-resolution　transmission　electron　microscopy　observations　and　ab　initio　calculations,　the　phase　transition　of　the　conductive　filament　shell　between　monoclinic　and　tetragonal　HfO2　is　proposed　to　depend　on　the　comprehensive　effects　of　Joule　heat　from　the　conductive　filament　current　and　the　concentration　of　oxygen　vacancies.　The　quasi-core-shell　conductive　filament　system　with　an　intrinsic　barrier,　which　prohibits　conductive　filament　oxidation,　ensures　the　extreme　scalability　of　resistive　switching　memristors.　This　study　renovates　the　understanding　of　the　conductive　filament　evolution　in　HfO2-based　memristors　and　provides　potential　inspirations　to　improve　oxide　memristors　for　nonvolatile　storage-class　memory　applications.