Abstract: 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%.