This paper presents the development of a new robust optimal decentralized PI controller based on nonlinear optimization for liquid level control in a coupled tank system.The proposed controller maximizes the closed-loop bandwidth for specified gain and phase margins,with constraints on the overshoot ratio to achieve both closed-loop performance and robustness.In the proposed work,a frequency response fitting model reduction technique is initially employed to obtain a first order plus dead time (FOPDT) model of each higher order subsystem.Furthermore,based on the reduced order model,a proposed controller is designed.The stability and performance of the proposed controller are verified by considering multiplicative input and output uncertainties.The performance of the proposed optimal robust decentralized control scheme has been compared with that of a decentralized PI controller.The proposed controller is implemented in real-time on a coupled tank system.From the obtained results,it is shown that the proposed optimal decentralized PI controller exhibits superior control performance to maintain the desired level,for both the nominal as well as the perturbed case as compared to a decentralized PI controller.