The driving factors of water vapor and energy exchange over an Inner Mongolian temperate desert steppe (Sunitezuoqi Temperate Desert Steppe Ecosystem Research Station) were investigated during the year of 2008, using eddy covariance, biotic and environmental data. The results showed that diurnal variation of energy balance components (net radiation (Rn); sensible heat flux (H); latent heat flux (LE); soil heat flux (G)) followed the expected single-peak shape. During daylight hours, H/Rn > G/Rn > LE/Rn, and at night the ratio G/Rn dominated the energy balance. On a daily basis, LE/Rn was relatively lower than H/Rn, even during the peak growth period. This energy partitioning could be explained by the typical dry climate and low vegetation cover of the desert steppe. The values of daily Rn were affected by the weather variation, especially during the rainy season when high day-to-day variation was observed. Energy balance components showed different seasonal patterns. The maximum monthly totals of Rn, H, LE and G occurred in July, May, June and June, respectively. On an annual scale, H was 58% and LE was 26% of Rn. Annual evapotranspiration (190.3 mm) was higher than the annual precipitation (136.3 mm), and got close to the annual average precipitation (183.9 mm). The maximum daily evapotranspiration rate was 3.8 mm d-1. As expected, G was a minor component of the annual energy balance (1%). Nevertheless, G was highly relevant on different time scales, acting as an "energy buffer" in the energy balance, i.e., the soil stored energy in the daytime and in the summer and released energy at night and in the winter, respectively. Precipitation profoundly affected the water vapor and energy exchange over the steppe, mainly by changing the soil water content (SWC). After precipitation events, the midday values of LE increased considerably, while H showed the opposite. The variation of daily evapotranspiration rates followed closely the variation of SWC during the growing season (from May 1st to October 15th), showing a direct relationship with SWC. On the daily scale, LE/Rn was effected by SWC, saturation water vapor pressure deficit (VPD), and leaf area index (LAI) during the growing season. Linear increase of LE/Rn with SWC and linear decrease of LE/Rn with VPD were observed. Additionally, quadratic curve variation of LE/Rn with LAI was shown. It was also found a LAI threshold of 0.2 m2/m2. When the value of LAI was less than 0.2 m2/m2, LE/Rn was driven by SWC alone; otherwise if LAI was more than 0.2 m2/m2, LE/Rn was co-driven by SWC and LAI. Decoupling coefficient was applied to assess degree of the coupling between the desert steppe and the atmosphere. Compared with other grassland ecosystems, the decoupling coefficient of this steppe was relative low (0.15 in growing season). Nevertheless, results showed that LE was mainly controlled by Rn during the peak-growth stage, because of the relatively high decoupling coefficient values. On the other hand, VPD was the main controlling factor during the early and later growth stages, due to the relatively low decoupling coefficient values.