Ammonia water was investigated as a new absorbent of the chemical absorption process for the removal of CO2 in flue gas. The suitable range of ammonia water concentration and CO2 loading (mol CO2/mol NH3) were decided in the point of view of CO2 absorption capacity and NH4HCO3 precipitation. The absorption capacity of CO2 and the precipitation of NH4HCO3 in liquid phase were calculated by the Pitzer model for electrolyte solution. The CO2 absorption capacity of the ammonia water over 5 molNH3/kgH2O was higher than that of conventional amine absorbent. The CO2 loadings where precipitation occurred were decided at various absorbent concentrations. Theses values were higher than 0.5 in the concentration range of 5-14 molNH3/kgH2O at 293, 313 K. The absorber for the removal of CO2 in flue gas could be operated without NH4HCO3 precipitation by using high concentration of ammonia water below these CO2 loading values. The optimum temperature of the ammonia water absorbent for removal of CO2 in flue gas was 297-312 K depending on the concentration of ammonia water.