In large controller-datapath systems, the switching activity of datapath is administered by controller. The unnecessary switching activity will cause more power consumption, and therefore the design of controllers (i.e. Finite State Machines, FSMs) will influence the whole power consumption of the systems. The state assignment and output encoding are the two major factors influencing the power of system under the hardware implementation of controllers. In this paper, we present an integer linear programming (ILP) method to solve the state assignment and output encoding problems. The purpose is to reduce switching activity such that the goal of power optimization can be achieved. It has not to reschedule the operations of datapath under timing and resource constraints and has no extra area overhead. In order to verify the effectiveness of our proposed ILP approach, we use this approach to implement several controller-datapath systems. Experimental results show that our proposed approach achieves an average of 30.513% power savings compared to the traditional area optimal synthesis tool, SIS, where power is not considered. Our proposed approach does not cause extra area overhead while achieving a significant power saving of systems.