This paper presents a mathematical estimator, named a 'software sensor' for predicting the NO x in the fluegas of power plants. The NO x prediction by the software sensor is used in the control loop NH 3 gas injection to the de-NO x device to reduce the NO x emission from the stack. By the use of the prediction in place of the actual measurement of NO x by the analyzer, the delay time in the control system is eliminated, and as a result the control performance can be improved, thus resulting in a reduction in operating cost.