This　paper　discusses　the　collapse　mode　of　thin　reinforced　concrete　(RC)　plates　subjected　to　blast　load.　To　extend　the　well　known　plastic-mode　method　to　analyze,　not　only　perfect-plastic　plates　,　but　also　RC　plates,　it　is　needed　to　investigate　the　effect　of　material　cracking　on　the　collapse　mode　because　the　plate　might　have　been　cracked　on　both　upper　and　lower　surface　before　the　plastic-mode　fully　develops,　creating　an　unexpected　type　of　collapse　mode　shape.　A　new　failure　mode　is　proposed　and　verified　by　numerical　analysis　in　this　paper.　The　new　mode　is　a　result　of　the　material　cracking　and　has　an　un-negligible　effect　on　the　reaction　mechanism　of　the　RC　plate　to　the　blast　load.