Regulating the amount of liquids is a critical condition in many manufacturing processes. The tanks are also so linked together that the levels communicate and display a nonlinear conduct. The sliding mode control (SMC) is used to regulate the level of the coupling tank structure. We initially developed a mathematical model for a nonlinear multi input single output system. A simulation to track a non-linear three tank system model is performed using MATLB / SIMULINK. The performance of SMC is compared to PID controller