In this paper, controller design for multivariable coupled tank system is presented. The control of Coupled tank system offers difficulty due to interaction between the controlled variables. By considering the interaction, this paper describes the design of decentralized PI controller for coupled tank system using Biggest Log modulus Tuning method (BLT). The decentralized controllers are designed based on the fully coordinated design method by assuming controller parameters obtained by employing Ziegler-Nichol’s tuning method with proper design parameters. The design parameters are then adjusted to ascertain the stability of the system. The effectiveness of the designed controller is evaluated and the results interpreted by simulation using MATLAB SIMULINK software.