The paper presents the method for correlation of the NRTL equation parameters used in VLE calculation. The method finds the global minimum and four local minima having the lowest values. It has been demonstrated that the set of parameters obtained for global minimum is not always the best for the prediction of equilibrium of ternary and quaternary mixtures. In such cases the a(ij) parameter value has the crucial importance. On the base of computational results it has been stated in which cases the optimal set of the NRTL equation parameters is the set with all three correlated parameters and when the one with aij 0.3 and only two parameters correlated.