An accurate method presented accounts for uncertain model parameters ill nonlinear process optimization problems. The model representation is considered in terms of algebraic equations. Uncertain quantity parameters are often discretized into a number of finite values that are then used in multiperiod optimization problems. These discrete values usually range between some lower and upper bound that call be derived from individual confidence intervals. Frequently, more than one uncertain parameter is estimated at a time from a single set of experiments. Thus, using simple lower and upper bounds to describe these parameters may not be accurate, since it assumes the parameters are uncorrelated. In 1999 Rooney, and Biegler showed the importance of including parameter correlation in design problems by, using elliptical joint confidence regions to describe the correlation among the Uncertain model parameters. In chemical engineering systems, however; the parameter estimation problem is often highly, nonlinear, and the elliptical confidence regions, derived from these problems may not be accurate enough to capture the actual model parameter uncertainty. In this work, the description of model parameter uncertainty is improved by, using confidence regions derived from the likelihood ratio test. It captures the nonlinearities efficiently and accurately, in the parameter estimation problem. Several examples solved show the importance of accurately capturing the actual model parameter uncertainty at the design stage.