Single　noble　metal　atoms　and　ultrafine　metal　clusters　catalysts　tend　to　sinter　into　aggregated　particles　at　elevated　temperatures,　driven　by　the　decrease　of　metal　surface　free　energy.　Herein,　we　report　an　unexpected　phenomenon　that　noble　metal　nanoparticles　(Pd,　Pt,　Au-NPs)　can　be　transformed　to　thermally　stable　single　atoms　(Pd,　Pt,　Au-SAs)　above　900　degrees　C　in　an　inert　atmosphere.　The　atomic　dispersion　of　metal　single　atoms　was　confirmed　by　aberration-corrected　scanning　transmission　electron　microscopy　and　X-ray　absorption　fine　structures.　The　dynamic　process　was　recorded　by　in　situ　environmental　transmission　electron　microscopy,　which　showed　competing　sintering　and　atomization　processes　during　NP-to-SA　conversion.　Further,　density　functional　theory　calculations　revealed　that　high-temperature　NP-to-SA　conversion　was　driven　by　the　formation　of　the　more　thermodynamically　stable　Pd-N-4　structure　when　mobile　Pd　atoms　were　captured　on　the　defects　of　nitrogen-doped　carbon.　The　thermally　stable　single　atoms　(Pd-SAs)　exhibited　even　better　activity　and　selectivity　than　nanoparticles　(Pd-NPs)　for　semi-hydrogenation　of　acetylene.