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Nitrous oxide (N2O) flux in the semi-arid Leymus chinensis (Trin.) Tzvel. grassland in Inner Mongolia, China was measured for two years (from January 2005 to December 2006) with the enclosed chamber technique. The measurements were made twice per month in the growing season and once per month in the non-growing season. To evaluate the effect of aboveground vegetation on N2O emission, the ecosystem N2O flux over the grassland was measured, and concurrently soil N2O flux was measured after the removal of all the aboveground biomass. The possible effect of water-heat factors on N2O fluxes was statistically examined. The ecosystem N2O flux ranged from 0.21 to 0.26?kg nitrous oxide-nitrogen (N2O–N) ha? 1 year? 1, indicating that the Leymus chinensis grassland of Inner Mongolia was a source for the atmospheric N2O. There was no significant difference between the ecosystem N2O flux and the soil N2O flux. The ecosystem N2O flux was under similar environmental control as the soil N2O flux. Soil moisture was the primary driving factor of the N2O fluxes in the growing season of both years; the changes in water–filled pore space (WFPS) of soil surface layers could explain 45–67% of the variations in N2O fluxes. The high seasonal variation of the N2O fluxes in the growing seasons was regulated by the distribution of effective rainfall, rather than the precipitation intensity. While in the non-growing season, the N2O fluxes were restricted much more by air temperature or soil temperature, and 83–85% of the variations of the N2O fluxes were induced by changes in temperature conditions.