Abstract: In order to improve the competitive power of air transportation, it is necessary to strengthen the analysis of market demand, especially on the choice behavior of passengers between air transportation and highway or railway transportation. In this paper, a mathematical model of travel cost is established to quantitatively describe the choice behavior of passengers, in which the major factors considered by passengers in their travel method choice, such as money, time, convenience and comfort, are expressed in the formof cost. Generally speaking, the higher the travel cost of a travel method is, the less possible it is to be chosen by passengers. A comparison between the costs by different travel methods is made on two typical air routes,i. e., Beijing to Shanghai and Chengdu to Chongqing. Finally, the methods for decreasing the travel cost of air transportation are briefly discussed.