Tunnel　with　large　slop　is　one　of　the　main　structures　of　the　urban　traffic　tunnels.　The　effect　of　the　slope　on　the　critical　velocity　is　the　key　factor　for　the　smoke　control　system　design　in　the　kinds　of　tunnels.　Centered　on　the　controversy　about　the　critical　velocity　in　titled　tunnel,　numerical　simulations　and　experimental　studies　by　a　1:8　small-scale　tunnel　mode　on　the　critical　velocity　in　these　tunnels　were　studied,　and　comparison　between　the　numerical　and　experimental　results　with　those　predicted　by　two　famous　models　due　to　Kennedy　and　Wu　&　Bakar　was　presented.　The　research　results　show　that　the　critical　velocity　for　the　downward　driving　in　a　titled　tunnel　is　larger　than　that　in　the　normal　level　tunnel,　and　it　increases　with　the　tunnel＇s　slope　increasing.　New　grade　factor　for　the　Kennedy　model　is　derived　based　on　the　research　results.　Wu　&　Bakar　model　is　recommended　to　be　used　to　calculate　the　critical　velocity　when　the　slope　of　tunnel　is　larger　than　5%.　©　2017,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.