Year-round eddy covariance flux measurements were made in a semiarid grassland in Mongolia during 2009-2014 to examine the interannual variability in net ecosystem CO₂ exchange (NEE) and its climatic controls. Annual cumulative NEE ranged from −20.9 to 8.1 g C m⁻² yr⁻¹, indicating that the semiarid grassland ecosystem can be a carbon source or a carbon sink, depending on climatic conditions. A correlation analysis showed that interannual variability in NEE was significantly related to precipitation and soil water content at 10 cm depth (SWC₁₀) in June and to the vapor pressure deficit (VPD) in July. In addition, the variation in gross primary production (GPP) in July contributed most to the interannual variation in NEE; and GPP in July was significantly related to June precipitation, June SWC₁₀, and July VPD. From these results, we concluded that precipitation during the early growing season and air dryness during the middle growing season were essential climatic factors regulating the interannual variation in NEE in this semiarid grassland.