Gaseous N losses via ammonia (NH₃) volatilization were measured during the 2005 and 2006 growing seasons in a long—term (17 years) experiment with five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha -1). We aim to understand the seasonal variations in ammonia production, the potential carry—over effects of previous grazing and the underlying regulating processes involved. It was found that rates of NH₃ volatilization varied seasonally, ranging from 0.88 to 3.52 g N ha-1d-1 during the measurement period, with higher values in spring and early summer and lower values in late summer and autumn. Soil pH value, NH-N concentration, moisture and bulk density exerted controls on NH₃ volatilization over the two growing seasons. However, the constraining effect of bulk density on NH₃ flux, driven by grazing, muted the supporting effect of above other three environmental factors. Even though no statistically significant effects of different grazing intensities on rates of NH₃ volatilization were generally found, grazed treatments tended to result in relatively higher cumulative NH₃ volatilization, and relatively lower soil N content, plant growth and productivity. It indicates that restoration of the degraded grazed grassland may be slower in comparison with the ungrazed grassland. The study on NH₃ volatilization in grassland ecosystem is meaningful in terms of its contribution to advancing our understanding of the role of N loss in limiting grassland processes and to drawing up grazing guideline and fertilizing regime for utilizing the resources rationally.