Stratified　vapor?air　mixtures　are　formed　owing　to　evaporation　after　crude　oil　leaks　in　confined　spaces.　The　concentration　distribution,　ignitability　of　the　stratified　vapor,　flame　propagation　in　it,　and　pressure　buildup　owing　to　combustion　must　be　investigated　to　obtain　knowledge　for　explosion　protection　and　mitigation.　In　this　study,　the　concentration　distribution　of　the　vapor　above　a　1.0-mm-thick　crude　oil　layer　in　a　horizontal　duct　was　measured,　and　the　effects　of　the　ignition　delay　(tig)　on　the　ignitability,　flame　behavior,　and　combustion　overpressure　were　investigated.　The　experimental　results　revealed　that　the　average　concentration　gradient　of　vapor　decreased　with　the　evaporation　of　the　crude　oil　layer.　The　stratified　vapor　could　be　ignited　from　10　to　50　min,　after　fuel　supply,　by　an　electric　spark　150　mm　above　the　oil　surface.　A　two-zone　flame　structure　including　a　blue　premixed　flame　and　yellow　diffusion　flame　was　observed,　and　the　flame　was　farther　from　the　oil　surface　as　tig　increased.　The　maximum　horizontal　speed　of　the　blue　flame　tip　and　overpressure　owing　to　combustion　initially　increased　and　then　decreased　as　tig　increased　from　10　to　50　min.　The　yellow　diffusion　flame　had　an　important　function　in　the　maximum　explosion　overpressure.　A　maximum　overpressure　of　530　kPa　was　measured　at　tig　=　25　min,　which　was　80　kPa　higher　than　that　of　stratified　gasoline　vapor　obtained　under　nearly　identical　conditions　in　previous　research.