Reducing　carbon　emissions　in　the　transport　sector　requires　technology　upgrades　to　engines　and　the　use　of　car-bon-neutral　fuels.　Ammonia　is　a　zero-carbon　renewable　fuel　that　complements　hydrogen.　This　study　is　based　on　a　Miller　cycle　spark　ignition　engine　using　ammonia-hydrogen　fuel.　The　effects　of　the　mixture　ratio　of　ammonia　and　hydrogen　under　stoichiometric　ratio　and　lean　burn　condition　on　the　performance　of　the　Miller　cycle　engine　are　focused.　First,　a　commercial　Miller-cycle　ICE　was　modified　to　allow　a　dual　fuel　supply　of　ammonia　and　hydrogen.　The　engine　was　run　steadily　at　1500　rpm　with　the　manifold　absolute　pressure　(MAP)　maintained　at　60　kPa,　and　the　ammonia　and　hydrogen　supplies　were　adjusted　to　achieve　different　mixing　ratios　and　excess　air　ratios.　The　test　results　showed　that　an　increased　ammonia　percentage　under　stoichiometric　ratio　conditions　significantly　improved　the　Braking　mean　effective　pressure　(BMEP)　and　Braking　thermal　efficiency　(BTE)　of　the　ICE.　Under　lean　burn　conditions,　the　changes　of　BMEP　and　BTE　are　small　with　the　increase　of　the　ammonia　ratio.　The　exhaust　temperature　of　the　ammonia-hydrogen　engine　is　relatively　low,　so　it　is　necessary　to　consider　matching　a　small　inertia　turbocharger.　Ammonia　ratios　above　80　%　by　volume　resulted　in　erratic　engine　oper-ation　and　even　misfires.　Ammonia-hydrogen　fuel　blends　lead　to　higher　Nitrogen　oxide　(NOx)　emissions.