An important aspect of an automated highway system design is the synthesis of an automated vehicle following system. Associated with automated vehicle following system is the problem of the stability of a string of vehicle, i.e., the problem of spacing error propagation, and in some cases, amplification upstream from one vehicle to another, due to some disturbance at the head of the string. In this paper, the novel method to determine the stability of a string of vehicle is established based on the vector Liapunov method. Based on the new results for the stability of string of vehicle, the controller is constructed by sliding mode control method. The stability domain of the controller parameters of the string of vehicles is enlarged.