We propose a car transportation system referred to as iCART (intelligent Cooperative Autonomous Robot Transporters). In this system, two robots approach the car autonomously, and lift up it for supporting its weight from right/left side respectively. Then, two robots transport the car in coordination. Collision-free trajectories for approaching and transporting the car are generated based on the potential field in the configuration space. In this paper, we describe details of the way to construct a configuration space, to apply a potential to the configuration space, and to generate a trajectory. Additionally, for generating the collision-free trajectory on the environment where two robots exist, repulsive potentials are applied to each robot. Described methods are integrated to iCART and experimental results illustrate the validity of the proposed method.