Flue gas NOx concentration was measured at the outlet of gas turbine (the inlet of de-NOx catalyst) of a 71 MWe pressurized fluidized bed combustor. The effect of operating parameters on NOx emission was approximated by assuming a linear combination of three independent parameters, concentration of O-2 in the flue gas, SO2 removal efficiency, and dense bed temperature. Sensitivity factors for different type of coals were obtained. The relationship between the sensitivity factors and fuel ratio (fixed carbon/volatile matter ratio) was obtained. NOx emission was well approximated by the present approximation, even for coals/coal mixtures that were not used to determine the sensitivity factors. NOx emissions for coal/coal mixtures and combustor operation under fly ash recycle conditions can be predicted approximately from the equation derived from other coals under conditions without fly ash recycle.