The effects of NOx, CO2, and reaction temperature on the SO2 removal process have been investigated in a fluidized bed reactor by using CaO/fly ash sorbent in order to demonstrate any available method to promote the calcium utilization rate and form valuable gypsum as a byproduct for the dry desulfurization process. It was found that NOx increased the selectivity of CaSO4 as the final product in the sorbent during the desulfurization process. The effect of NOx on SO2 removal rate was also investigated and results showed that the presence of NOx enhanced the SO2 removal rate, even when mole ratio of NO/SO2 was less than 1.0. It was deduced that the reaction mechanism involved NOx-related chain reactions. On the basis of FTIR analysis of reactions product, the chain reaction mechanism could be outlined that Ca(NO3)(2) was formed from the reaction of Ca(OH)(2) with NO2, and then Ca(NO3)(2) reacts with SO2 to form CaSO4 + NO. On the other hand, it was found that the CO2 negative effect on SO2 removal was reduced with NOx presence.