Ammonia volatilization was measured with the continuous air flow enclosure method in a paddy field at the Experimental Farm of Jurong Agricultural School in Zhenjiang hilly region, and its main influencing factor was investigated.The result showed that ammonia volatilization was obvious in paddy field after fertilisation with urea at the transplanting, tillering and booting stages of the rice crop.The result also showed that there was a visible ammonia volatilization which varied with the growth period and that the highest NH₃ loss rate occurred following the first top-dressing (19~26 days after rice transplanting), and the lowest NH₃ loss rate following the basal fertilisation (only 1%~3.8% of the N applied).The annual variation in ammonia volatilization was also observed due to the climate factors.An evident diurnal variation in flooding water pH and soil pH was observed after fertilization, and affected the am monia volatilization significantly.The rice straw application had a dissimilar effect which varied with the growth period.Total NH₃ loss over the whole rice-growing season increased with the N dose, but little difference was observed in the ratio of NH₃ loss to total N applied.