Abstract: Ammonia volatilization was measured with the enclosure method in the laboratory in Japanese International Research Center for Agricultural Sciences (JIRCAS) in 2000. The object was to test the effect of air flow-rate, temperature and application method of urea on ammonia volatilization from paddy soils. The results showed that ammonia volatilization increased with the increase of air flow-rate and temperature. The peak of ammonia volatilization was 5~8 days after urea applied. There was almost not any ammonia volatilization on the 18th day after urea applied. Tillage after application decreased ammonia volatilization. Application CaCO3 before urea increased largely ammonia volatilization because the soil pH increased. The accumulated ammonia loss in the treatments was 26.4%~57.3%. So, ammonia volatilization is the main loss way of urea in paddy soils.