The fluxes of N2O emission from and CH4 uptake by the typical semi-arid grasslands in the Inner Mongolia, China were measured in 1998-1999. Three steppes, i.e. the ungrazed Leymus chinensis (LC), the moderately grazed Leymus chinensis (LC) and the ungrazed Stipa grandis (SG), were investigated, at a measurement frequency of once per week in the growing seasons and once per month in the non-growing seasons of the LC steppes. In addition, four diurnal-cycles of the growing seasons of the LC steppes, each in an individual stage of grass growth, were measured. The investigated steppes play a role of source for the atmospheric N2O and sink for the atmospheric CH4, with a N2O emission flux of 0.06-0.21 kg N ha(-1) yr(-1) and a CH4 uptake flux of 1.8-2.3 kg C ha(-1) yr(-1). Soil moisture primarily and positively regulates the spatial and seasonal variability of N2O emission. The usual difference in soil moisture among various semi-arid steppes does not lead to significantly different CH4 uptake intensities. Soil moisture, however, negatively regulates the seasonal variability in CH4 uptake. Soil temperature of the most top layer might be the primary driving factor for CH4 uptake when soil moisture is relatively low. The annual net emission of N2O and CH4 from the ungrazed LC steppe, the moderately grazed LC steppe and the ungrazed SG steppe is at a CO2 equivalent rate of 7.7, 0.8 and -7.5 kg CO2-C ha(-1) yr(-1), respectively, which is at an ignorable level. This implies that the role of the semi-arid grasslands in the atmospheric greenhouse effect in terms of net emission of greenhouse gases (CO2, CH4 and N2O) may exclusively depend upon the net exchange of net ecosystem CO2 exchange.