Abstract: The purpose of this study was to set up a model to estimate the travel cost of private car in big cities considering traffic congestion. Furthermore, taking Beijing as an example, the dynamic change of travel cost with speed caused by traffic congestion in Beijing was estimated. The results indicate that the total travel cost and the proportion of external cost increase with the decrease of road network speed. When the speed of road network is 35 km/h, there is no congestion, the average travel cost of private cars in Beijing is 7.42 CNY/（veh·km）, of which the proportion of external cost accounts for about 1/3 of the total cost; However, when the velocity of road reduces to 25 km/h, traffic congestion appears, the average travel cost increases to 11.86 CNY/（veh·km）, and the proportion of external cost is nearly 1/2.

Key words: travel cost, private car, traffic congestion, external cost