This paper develops a procedure for designing a self-tuning-controller (STC) with the capability of controlling single-input-single-output (SISO) nonlinear processes operating under the presence of nonlinear disturbances. Such processes can be represented by nonlinear differential equations, where the nonlinear terms complicate the analytical aspects of modeling and controller design. However, based on the corresponding linear process model, we can form a discrete Self-Tuning Control System (STCS), in which is possible to employ linear control algorithms. Modeling errors should be corrected by the estimation procedure of the STCS. An application is provided to demonstrate the validity of the design approach: controlling a nonlinear position process exposed to nonlinear disturbances.