The task of this investigation is to design a controller that has a strong robustness in various operating conditions. A new structure of state feedback assisted classical control (SFACC) that uses a differential lag compensator in the inner classical control loop is proposed to improve the robustness of original SFACC. The linear quadratic Gaussian with loop transfer recovery (LQG/LTR) at the plant output is employed to design the robust controller in the outer control loop. A comparison of the performance and robustness between the gain-insensitive controller and an existing LQG/LTR controller is made by nonlinear simulations. The proposed gain-insensitive LQG/LTR controller can give satisfying performance for both reactor power and coolant temperature over a wide range of reactor operations.