Abstract: A new control law, the state differential feedback control, which was applied to design the control system of a reactor in the load-following mode, was presented in this paper. The design processor depended on the equivalence between two control systems, which included the integral control for control rod position and the state differential feedback control for control rod speed, which was obtained as a theorem. The simulation results show that the state differential feedback control system has a good load-following capability of a reactor.