Abstract: Four adjacent sites with different land-use histories were selected in the agro-grassland ecotone of Inner Mongolia over three growing seasons from 2008-2010. The sites included grassland and cropland cultivated for 5,0 and 50 years after their conversions. N₂O flux measurements were obtained using the closed-chamber method and performed continuously from April to October in 2008,9 and 2010, respectively. The results indicated significant differences of N₂O emissions between cropland and grassland in 2008 (F=53.8, P<0.001), 2009 (F=17.3, P<0.001) and 2010 (F=153.0, P<0.001). Seasonal cumulative N₂O emissions were 87.6,1.8, and 211.0 mg/m² during seven months for the growing seasons in 2008,9 and 2010. From 2008-2010, the cumulative seasonal N₂O emissions of the cropland soils that were 5 and 10 years old were 10%-50% less than those of the grasslands, and the seasonal N₂O emissions from 50 year-old cropland soil were 10%-30% greater than those of the grasslands. Thus, N₂O emissions increased with the growth of the cropland age from 5 to 50 years. The cumulative seasonal N₂O emissions of the grassland and cropland soils could be quantitatively predicted from the soil NH₄⁺-N and moisture concentrations (R²=0.58, P<0.01).