The performance of a commercial circulating fluidized bed boiler in the Yeosu thermal power plant, which has been operating since October 2011 by KOSEP, has been evaluated by using the IEA-CFBC model. To validate the calculation procedure of the model, the calculated results were compared with the operation values such as the temperatures, pressures, emissions of SO2 and NO, particles size distribution and unburned carbon fraction of the CFB boiler at a certain actual condition. The calculated results were comparable to measured values from the CFB boiler, so these could conform to acceptable formats with a good accuracy. The effect of the primary to secondary air ratio on the performance of the CFB boiler was also determined. As the primary air ratio increased, the solid fraction and temperature in the furnace freeboard increased. As a result, the solid circulation rate and the heat absorption in the furnace increased with increasing the PA ratio. In the case of the amount of heat absorption, the wall tube of the furnace absorbed much more generation heat in the furnace than the wing wall tube. The SO2 emission decreased due to increase of the limestone hold up in the furnace, and the combustion efficiency somewhat increased with increasing the PA ratio. Therefore, from these results, we could expect to control the boiler performance such as the furnace temperature, steam temperatures of superheater or reheater, gaseous emissions and combustion efficiency through the changing the PA ratio of the CFB boiler.