Ammonia　(NH3)　synthesis　is　a　structure-sensitive　reaction,　where　a　minute　variation　of　catalyst　structure　would　lead　to　a　dramatic　change　of　activity.　To　date,　the　roles　of　Ru　size　on　NH3　synthesis　still　remain　elusive,　and　it　remains　a　major　challenge.　Herein,　a　series　of　Ru　catalysts　with　sizes　ranging　from　1.4　to　5.0　nm　were　prepared　by　colloidal　Ru　deposition,　which　allows　the　investigation　of　size　influence　on　NH3　synthesis.　Through　analyses　of　the　geometric　and　electronic　properties　of　variable-sized　Ru　particles,　it　shows　that　the　reduction　of　Ru　particle　size　could　increase　the　proportion　of　corner　sites　while　decrease　that　of　terrace　sites,　which　can　decrease　the　work　function　and　thus　promote　N2　for　its　activation.　Isotopic　investigations　show　that　the　decrease　of　dissociative　contribution　but　the　increase　of　associative　contribution　as　Ru　particle　size　was　decreased　from　5.0　to　1.4　nm.　Meanwhile,　the　NH3　synthesis　rate　increases　when　the　size　of　Ru　catalysts　decreases　from　5.0　to　1.4　nm.　Using　a　suite　of　elaborate　characterizations,　we　have　observed　that　1.4　nm　Ru　nanoclusters　(NCs)　with　ample　corner　sites　are　conducive　to　activate　N2　via　an　associative　route.　As　a　result,　the　1.4　nm　Ru　NCs　catalyst　shows　the　highest　NH3　synthesis　rate　(up　to　17.13　mmolNH3　gcat−1h−1)　at　400　and　1　MPa.　©　2022　Elsevier　Inc.