Electrochemically　splitting　water　greatly　demands　low-cost,　high-performance　oxygen　evolution　reaction　(OER)　catalysts.　However,　synthesizing　size-controllable　magnetite　(Fe3O4)　nanoclusters　as　high-performance　electrocatalysts　poses　a　great　challenge,　as　the　size　significantly　affects　their　properties.　Here,　we　present　a　modified　polyol　process　for　tuning　the　size　of　Fe3O4　nanoclusters　in　the　presence　of　an　ionic　liquid.　The　average　size　of　the　as-prepared　nanoclusters,　which　are　composed　of　nanocrystals　with　a　fixed　size　(∼5.6nm),　can　be　effectively　adjusted　from　236nm　to　63nm　with　the　increase　of　the　added　volume　of　ionic　liquid　from　0μL　to　20μL.　The　nanoclusters　were　successfully　used　as　catalysts　for　OER,　and　those　with　a　size　of　63nm　showed　the　highest　catalyst　activity,　possessing　336mV　of　overpotential　under　20mA　cm−2　of　current　density.　The　result　of　the　double-layer　capacitance　analysis　firmly　confirms　that　the　large　electrochemical　active　area　of　the　magnetite　nanoclusters　provides　more　catalytic　active　sites,　which　leads　to　higher　catalyst　activity.　Furthermore,　the　magnetite　nanoclusters　exhibit　stable　catalyst　performance　and　can　maintain　oxygen　evolution　activity　for　a　long　time.　This　process　provides　a　new　strategy　for　the　size-controlled　synthesis　of　other　metal　oxide　nanoclusters　that　could　be　an　effective　substitute　for　OER　noble　metal　catalysts.　©　2025　World　Scientific　Publishing　Company.