With　high　energy　density　both　by　weight　and　volume,　ammonia　(NH3)　is　a　promising　hydrogen　carrier.　Furthemore,　NH3　has　a　mature　industrial　background,　and　in　liquid　form　storage　and　transportation　is　not　a　problem.　Adding　the　merit　of　zero　CO2　emission,　NH3-to-power　by　direct　ammonia　solid　oxide　fuel　cells　(DA-SOFCs)　is　an　acceptable　strategy　to　facilitate　hydrogen　usage.　Nonetheless,　to　achieve　efficacy,　a　high　compatibility　between　operating　temperature　and　catalytic　materials　for　NH3　decomposition　is　needed.　In　this　work,　we　developed　a　tubular　DA-SOFC　with　an　output　power　capability　of　＞　3　W.　By　combining　experimental　measurements　and　multi-physics　simulation,　we　comprehensively　studies　the　related　intrinsic　processes.　Based　on　experimental　data,　we　developed　a　two-dimensional　multi-scale　electro-thermo　model　of　tubular　DA-SOFC.　Separately　we　evaluated　the　effects　of　inlet　fuel　gas　composition,　inlet　flow　velocity,　operating　temperature,　and　operating　voltage　on　the　rate　of　NH3　catalytic　decomposition　and　H-2　electrochemical　oxidation,　as　well　as　on　NH3　conversion,　H　atom　utilization,　and　electrical　efficiency　of　the　tubular　DA-SOFC.　The　results　suggest　that　high　H　atom　utilization　could　be　realized　by　matching　the　rate　of　NH3　decomposition　with　that　of　H-2　electrochemical　oxidation.　It　was　observed　that　with　the　decrease　of　temperature,　the　rate　of　H-2　oxidation　decreases　more　rapidly　than　that　of　NH3　decomposition,　suggesting　that　the　flow　velocity　of　NH3　should　be　appropriately　lowered　to　optimize　H　atom　utilization.　Finally,　we　established　a　correlation　between　H　atom　utilization,　operating　voltage,　and　electrical　efficiency　for　synergistic　optimization　of　operating　conditions.　At　0.7　V　and　800　celcius,　the　tubular　DA-SOFC　fueled　with　NH3　of　27　mL.min(-1)　is　capable　of　offering　3.2　W,　displaying　an　efficiency　of　60%.　Compared　to　that　of　a　tubular　H-2-SOFC　(only　51%　efficiency),　the　efficiency　is　significantly　higher　on　the　basis　of　equal　voltage　and　fuel　utilization　ratio.　The　outcome　of　the　present　study　demonstrates　the　potential　of　tubular　DA-SOFC　as　a　device　for　high-efficiency　power　generation.