Herein,　a　versatile amorphous-to-crystalline　transformation　(ACT)　strategy　is　described,　to　furnish　various　metals　(Cu,　Co,　Ni,　Cs,　and　Y)　based　atomically　dispersed　catalysts　(ADCs)　on　thin　holey　2D　Al2O3.　This　approach, which　involves　adjusting　reactant　stoichiometry,　enables　the　continuous　modulation　of　particle　size　of　ADCs　at　an　atomical　level,　giving　the　ultrafine　products　as　single　atom,　cluster,　or　nanoparticle　catalysts.　This　synthesis　method allows　a　straightforward　analysis　and　comparison　of　the　reactivity　of　the　ADCs　catalysts　based　on　the　dispersion　of　the　active　metal.　In　the　case　of　Cu,　the　cluster　catalyst　0.2-Cu/Al2O3　outperforms　the　single　atom　catalyst　0.1-Cu/Al2O3　and　the　nanoparticle　catalyst　0.3-Cu/Al2O3　in　the　oxidation　of　refractory　organic　molecules,　thanks　to　its　superior　electron　transport　and　surface　adsorption　properties.　These　findings　are　well　supported　by　the　Density　functional　theory　(DFT)　calculations.　Additionally,　this　method　facilitates　the　preparation　of　atomic　clusters　with　a　very　small　size　of　0.5–1 nm,　composed　of　just　a　few　atoms,　as　exemplified　by　Ni-based　ADCs.　The　developed　synthesis　enriches　the　library　of　ADCs　and　demonstrates　the　potential of　amorphous-to-crystalline　transformation　in　creating　advanced　ultrasmall　functional　materials.　©　2025　Wiley-VCH　GmbH.