Abstract: This paper presents a new technique of adaptive and optimal PI controller with robustness. On the basis of process input and output data of normal operation in the control loop, important frequency responses of process are estimated by signal decomposition and analysis in frequency domain. Then, PI control parameters are calculated to satisfy both the robusess specification λ and optimization of load disturbance rejection. The control performance can be easily regulated to meet the practical requirements by changing robustness specification λ. The adaptive procedure of PI controller does not need any prior knowledge of the process and previous controller while the control loop is still in the normal operation. Simulation example is given to show both effectiveness and feasibility of the adaptive PI controller.