Motivated　by　controversy　of　the　grade　correction　factor　in　the　critical　velocity　model　recommended　in　most　tunnel　design　codes,　the　effect　of　slope　on　the　critical　velocity　in　a　tilted　tunnel　was　studied　by　simulation　and　experimental　methods.　A　1:8　reduced-scale　tilted　tunnel　model　was　built　for　the　experimental　studies.　When　a　tunnel　is　tilted,　smoke　movement　is　asymmetrical　about　the　fire　source　due　to　buoyancy　along　the　longitudinal　tunnel　axis.　In　this　paper　only　the　worse　scenario　of　downhill　longitudinal　ventilation　to　suppress　smoke　back　layering　uphill　is　considered.　Both　the　simulation　and　experimental　results　in　the　present　study　show　that　the　grade　correction　factor,　referred　to　the　critical　velocity　for　horizontal　tunnel　in　Danziger　and　Kennedy＇s　model,　depends　on　the　slope　in　a　linear　manner.　The　values　of　the　critical　velocity　in　the　present　study　agree　well　with　those　predicted　by　the　model　of　Wu　and　Bakar　(2000),　but　are　somewhat　higher　than　the　values　predicted　by　the　model　of　Danziger　and　Kennedy　(1982).