Hierarchy control strategy and ant colony optimization are proposed for coordinated ramp control. The macroscopic model to describe the evolution of freeway traffic flow is firstly established. Then the coordinated ramp control system is designed. There are two layers in this coordinated control system: the coordination control layer to select traffic models, to adjust the model parameters, and to determine the desired traffic density in each freeway section according to the current traffic status; and the direct control layer to keep the actual values of state variables in the vicinity of the desired state points via PI controllers. Ant colony algorithm is used to find the optimal PI control parameters. Simulation results show that the control system is of good performance. It can eliminate traffic jams, maintain traffic flow stability, and make vehicles travel more efficiently and safely.