Ammonia volatilization was assessed with a nondisturbing micrometeorological technique after urea application in three different periods in paddy fields (Gleyi stagnic Anthrosol) of the Taihu Lake region. And some factors (such as climate, NH₄⁺-N concentration in floodwater, et al.) influencing the ammonia loss were also studied. The results indicated that the ammonia volatilization loss from the applied urea in the paddy fields varied between 18.6% and 38.7% of the total N applied each time. The peak of ammonia loss occurred within the first week after the application, amounting to 80.7%~94.3% of the total ammonia loss in each period. The highest ammonia volatilization flux from the paddy fields occurred within the first 24~72 hours after the application of basal fertilizer, tillering fertilizer and ear bearing fertilizer, being N 7.49, 11.17, and 5.52 kg hm^-2d^-1, respectively in Treatment HN and N 3.01, 6.79, and 1.72 kg hm^-12 d^-1 in Treatment LN. The ammonia loss from the tillering fertilizer was the highest and from the earbearing fertilizer was the lowest. Factors influencing ammonia volatilization loss varied in effect in different rice growing stages. Significantly positive correlationship was observed between NH₃ loss and NH₄⁺-N concentration in the surface water in the paddy fields in all the three fertilization perio.