Subway　tunnel　is　a　kind　of　limited　ventilation　space　with　a　large　slenderness　ratio.　Backdraft　may　take　place　under　certain　conditions　in　tunnel　fire.　Some　experiments　have　been　conducted　to　the　tunnel　fire　backdraft　by　the　1/8　reduced-scale　model　experimental　apparatuses.　The　results　indicated　that　the　mass　con-centration　of　the　volatilized　fuel　is　the　key　parameter　determining　the　occurrence　of　backdraft　in　subway　tunnel　fire,and　the　critical　value　of　the　mass　concentration　was　equal　to　8.78%　in　the　single-tunnel　with　natural　ven-tilation.　And　the　critical　value　can　be　obtained　at　the　same　time　from　the　flammability　diagram　of　fue1,which　was　8.66%.　Under　the　condition　of　natural　ventilation,　the　characteristics　of　backdraft　occurred　in　the　single-tunnel　was　almost　the　same　as　that　in　compartment,　while　their　values　were　different.　Therefore,　the　flamma-bility　diagram　of　fue1can　be　used　to　find　the　range　of　the　critical　values　in　a　real-scale　subway　tunnel,　which　will　play　an　important　role　in　the　fire　disaster　prevention.