CFD　simulation　tests　were　carried　out　to　study　the　asymmetric　flow　phenomenon　caused　by　the　combination　effect　of　tunnel　slope　and　longitudinal　fire　location　in　a　naturally　ventilated　tunnel.　The　result　shows　that　the　tunnel　slope　and　the　longitudinal　fire　location　can　both　create　the　induced　longitudinal　flow　solely.　The　longitudinal　flow　within　tunnel　induced　by　the　stack　effect　caused　by　the　tunnel　slope　is　normally　unidirectional,　flowing　uphill　and　the　speed　increases　gradually　with　the　tunnel　slope.　While　the　longitudinal　flow　induced　by　the　thermal　pressure　difference　caused　by　the　uneven　distribution　of　upstream　and　downstream　smoke　transportation　can　be　bidirectional,　which　depends　on　the　longitudinal　fire　location　(also　known　as　the　downstream　and　upstream　tunnel　length　difference,　ΔL　=　Ldown　−　Lup).　Therefore,　the　induced　longitudinal　flow　under　the　combination　effect　of　tunnel　slope　and　longitudinal　fire　location　is　very　complex.　For　tunnels　going　uphill　from　left　(upstream)　to　the　right　(downstream)　portals,　the　two　effects　are　positively　added　when　the　fire　is　located　at　the　upstream　tunnel　(ΔL　＞　0),　while　the　two　effects　are　counteracted　when　the　fire　is　located　at　the　downstream　(ΔL　＜　0).　To　quantify　the　strength　of　asymmetric　flow　caused　by　the　two　effects,　an　empirical　equation　of　mass　flow　rate　of　induced　longitudinal　flow　is　proposed.　Meanwhile,　a　model　to　predict　the　smoke　back-layering　length　under　the　two　effects　is　also　proposed　and　validated　by　former　experimental　data.　A　prediction　model　on　the　upper　critical　fire　position　where　the　two　effects　are　completely　canceled　out　is　proposed.　Another　critical　position　(the　lower　critical　fire　position)　where　the　smoke　flow　can　achieve　unidirectional　transportation　in　an　inclined　tunnel　under　natural　ventilation　conditions　is　deduced　as　well.　©　2024　Elsevier　Ltd