Direct　ammonia　solid　oxide　fuel　cells　(DA-SOFCs)　have　triggered　great　interest　due　to　their　efficient　power　generation　from　ammonia　directly.　However,　the　compatible　match　of　ammonia　decomposition　and　electrooxidation　in　the　DA-SOFCs　remains　greatly　challenging　due　to　their　endo/exothermic　properties.　Herein,　multi-sizes　tubular　DA-SOFCs　were　systematically　investigated　for　performance　evaluation　of　power　output　and　ammonia　decomposition.　Accordingly,　a　multi-scale　electro-thermo　model　for　tubular　DA-SOFC　was　established　to　intensify　the　synergy　between　complex　physical–chemical　processes　and　geometry.　With　the　combination　of　experimental　work　and　simulations,　the　effects　of　operating　conditions　and　geometry　were　comprehensively　evaluated.　Significantly,　the　rates　of　ammonia　decomposition　and　electrooxidation　could　be　effectively　matched　through　the　optimization　of　operating　conditions.　The　geometric　design　further　enables　the　temperature-zoning　of　the　two　processes,　competently　enhancing　the　thermal　coupling　between　them.　Conclusively,　the　correlation　equations　linking　the　operating　conditions,　geometry　and　electrical　efficiency　were　proposed　for　the　scaling-up　of　tubular　DA-SOFCs　unit.　The　tubular　DA-SOFC　achieves　3.5　W　with　60%　electrical　efficiency,　and　performed　a　satisfactory　stability　for　over　330　h.　This　study　provides　guidance　for　oriented　design　of　tubular　DA-SOFCs　with　high　electrical　efficiency.　©　2025　American　Institute　of　Chemical　Engineers.