Low-temperature　direct　ammonia　fuel　cell　via　ammonia　oxidation　reaction　(AOR)　is　one　of　the　most　attractive　ways　for　ammonia　utilization.　The　Pt–Ir　alloyed　catalysts　have　been　proven　greatly　efficient　in　AOR　to　overcome　the　sluggish　kinetics　and　complex　multi-electron　processes;　however,　it　still　remains　challenging　for　further　improvement　because　of　the　complexity　of　alloyed　types　and　unclear　fundamental　understanding.　Herein,　we　systematically　fabricated　a　series　of　PtxIry/XC-72　catalysts　via　tuning　the　Pt/Ir　compositions　and　these　catalysts　demonstrate　significant　composition-dependent　behaviours　in　AOR,　whereas　the　Pt　favours　increasing　current　densities　and　the　Ir　facilitates　reducing　onset　potentials.　Importantly,　the　actual　surface　Pt/Ir　atomic　configuration　is　totally　different　with　the　classic　view　for　homogeneous　alloys　by　combining　catalytic　and　characteristic　analyses.　The　Pt　prefers　to　be　segregated　on　the　topmost　surface.　Then　the　quantified　correlations　between　current　density/onset　potential　and　Pt　composition　in　Pt　−　Ir　nanoparticles　(NPs)　were　further　established　and　discussed.　The　onset　potential　of　AOR　is　significantly　reduced　by　Ir　incorporation　while　it　is　insensitive　to　Ir　contents.　Accordingly,　the　well-studied　Pt5Ir5/XC-72　catalyst　was　further　employed　in　an　alkaline　exchange　membrane　fuel　cell,　exhibiting　a　peak　power　density　(230.0　mW　cm−2).　Our　work　provides　insights　into　the　effect　of　Pt/Ir　compositions　on　AOR,　with　the　goal　of　catalysts　engineering　for　AOR　and　ammonia　fuel　cells.　©　2023