In this paper, we develop predicting dynamic trajectory of surround vehicles for adaptive cruise control. The driver can get feeling of discomfort in current adaptive cruise control system when the vehicle cut in or cut out. We propose primary target selection algorithm in order to improve ride comfort. The algorithm for primary target selection of surround vehicles is developed by using radar. The simulation results in this paper indicate that smoother tracking and smoother acceleration. The performance of the proposed predicting method is validated via simulations implemented in CarSim and Matlab/Simulink.