Stratified　methane-air　mixtures　are　formed　in　the　early　stages　of　methane　leakage　in　confined　spaces,　and　an　explosion　may　occur　if　a　source　of　ignition　were　to　appear　at　the　appropriate　time　and　location.　However,　this　has　not　been　elucidated　thus　far.　The　aim　of　this　study　is　to　measure　the　concentration　gradient　of　methane　injected　from　the　top　surface　of　a　duct,　and　to　investigate　the　effects　of　ignition　delay　(tig),　which　is　the　time　interval　from　methane　leakage　to　ignition,　on　flame　behavior　and　explosion　overpressure.　Experimental　results　show　that　tig　significantly　affects　flame　behavior　in　stratified　methane-air　mixtures,　including　the　flame　shape　and　speed.　The　stratified　methane-air　mixtures　cannot　be　ignited　at　tig　≤　3　min.　The　horizontal　flame　speed　and　maximum　overpressure　(Pmax)　first　increase　as　tig　increases　from　4　min　to　15　min　and　thereafter　remain　nearly　unchanged　with　further　increase　in　tig.　The　value　of　Pmax　at　tig　in　the　range　15–25　min　is　approximately　that　observed　for　homogeneous　methane-air　mixtures.　Tulip　flames　are　observed　at　tig　≥　10　min.　Diffusion　and　convection　flames,　which　follow　the　leading　premixed　flame　front,　are　only　formed　at　tig　≤　6　min.　©　2019