In this work a nonlinear PID sliding surface is used to design a Sliding Mode Controller. A linear sliding surfaces have certain disadvantages; for example, the magnitude of the control input required to keep the system states on the sliding surface usually increases in direct proportion to the magnitude of the tracking error. Thus, the nonlinear sliding surface may provide better performance if the nonlinearity is appropriately selected. The proposed controllers are applied to a nonlinear mixing tank to test its performance. It have been compared against a PID controller and a Sliding mode control through the simulations and performance indices, ISE, TVu, maximum overshoot, and settling time.