Identifying　optimal　Ru　size　in　NH3　synthesis　can　improve　reaction　activity　and　maximize　the　utilization　of　Ru　to　reduce　catalyst　cost.　However,　previous　researches　are　focused　on　large　Ru　particle　size　(≥2　nm)　while　that　below　2　nm　in　NH3　synthesis　is　unclear.　Here　we　synthesized　a　series　of　Rux/BaCeO3　with　different　Ru　sizes　(x　=　1.1–3.0　nm)　through　size-controlled　Ru　colloid.　With　the　decrease　of　Ru　size,　NH3　synthesis　rate　over　Ru1.1/BaCeO3　increases　to　19.4　mmol　gcat−1h−1　at　400　°C　and　1　MPa,　which　is　5.7　times　that　of　Ru3.0/BaCeO3　and　superior　to　most　of　Ru-based　catalysts　previously　reported.　It　reveals　that　the　reduction　of　Ru　size　enhances　the　generation　of　Ce3+　and　oxygen　vacancies　in　BaCeO3,　which　can　donate　electron　to　Ru　centers　and　promote　N2　dissociation.　Moreover,　the　small　Ru　size　enhances　hydrogen　spillover　from　Ru　to　BaCeO3　to　alleviate　hydrogen　poisoning,　resulting　in　efficient　NH3　synthesis.　©　2021　Elsevier　Inc.