The effects of acetic acid injection and operating conditions on NO emission were investigated in a pilot scale vortexing fluidized bed combustor (VFBC), an integration of circular freeboard and a rectangular combustion chamber. The dimension of the freeboard is 0.75 m I.D. and 4.6 m in height. The cross section of the combustion chamber is 0.8 x 0.4 m(2), and the height of the combustion chamber is 1.47 ill. The secondary air injection nozzles were installed tangentially at the bottom of file freeboard. Coal was used as the fuel. Silica sand was employed as the bed material. Acetic acid was used as the reductant to reduce NO emission. Operating conditions, such as the stoichiometric oxygen in the combustion chamber, the bed temperature and the injecting location of acetic acid, were determined by means of response Surface methodology (RSM). which enables the examination of parameters with a moderate number of experiments. Ill RSM, NO emission concentration after acetic acid injection and NO removal percentage at the exit of the VFBC are used as the objective function. The results show that the bed temperature has it more important effect on the NO emission than the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber. Meanwhile. the injecting location of acetic acid and the stoichiometric oxygen in the combustion chamber have a more important effect oil the NO removal percentage than the bed temperature. NO emission can be decreased by injecting the acetic acid into the combustion chamber, and NO emission decreases with the. height of the acetic acid injecting location above the distributor. On the other hand, NO removal percentage increases with the height of the acetic acid injecting location, and NO emission increases with the stoichiometric oxygen in the combustion chamber and the bed temperature. NO removal percentage increases with the stoichiometric oxygen, and increases first. then decreases with the bed temperature. Also, a higher NO removal percentage could be obtained at 850 degrees C.