Freeway is a key component of urban transportation system.With the rapid growth of traffic volume,traffic congestion occurs frequently.Ramp metering is one of the most effective measures to combat traffic congestion on urban freeway.The fundamental function of ramp metering of urban freeway is expounded,and the key factors which determine the metering rate are pointed out.The impacts of ramp metering on freeway,surface road and traffic corridor are analyzed completely.Meanwhile,corresponding control strategies are put forward.