Ammonia (NH₃) volatilization from a silty clay loam paddy soil applied with non, normal urea and coated urea, respectively, under transplanting practice in Miryang, Korea from 2002 and 2003 was simulated by a Water and Nitrogen Management Model (WNMM) compared against the field measurement. Based on the data of the in-situ field measurements, the NH₃ volatilization during whole rice growing season was 1.46% of applied N for coated urea compared to 6.04% for normal urea. In WNMM, the bulk aerodynamic approach was used to simulate the NH₃ volatilisation from paddy water surface. As a result, the WNMM predictions of NH₃ volatilization during these two rice growing seasons (2002 and 2003) were satisfactory to distinguish the applications of two urea forms, but slightly less than the field measurements. In addition, the R2 for the correlation between predicted and observed NH3 volatilisation during the calibration year (2002) is 0.53 less than that of the application year (2003), 0.86. The reason of the above difference may be explained by huge rainfalls happening during the calibration year. However, there is an uncertainty about the dynamics of pH in paddy water caused by the development of blue algae in the field, which needs more accurate simulation because the NH₃ volatilisation from paddy water is mainly driven by water pH.