Terrestrial Ecosystems Transect is one of the noticeable innovations for the core item “Global Change and Terrestrial Ecosystem (GCTE)” of the International Geosphere-Biosphere Programme (IGBP). Northeast China Transect (NECT) was put forward and established by Zhang Xinshi in 1991, and was formally named NECT by IGBP in 1993. The NECT is placed along the latitude 43 30′N between 112°E to 130°30′E ,and approximately 1600 km length. It is basically a gradient driven by precipitation in middle latitude of the temperate zone. The NECT extends from the needle-broad-leaved mixed forest to steppe of temperate zone. At present, the NECT had become an important platform on all-around global change research of multitudinous subjects in China. This paper was a part of the third all-around NECT investigation. Leymus chinensis is an important edificator in the temperate zone steppe of China. Its distributing area is accordant with the area of Eurasia steppe. The NECT runs through its distributing center in China. Running about 1000km (from 125 to 115), the NECT consists of meadow and typical steppes (Leymus Chinensis with Stipa baicalensis, S. grandis and S. krilovii), meso-xeric dene steppe (L. chinensis with Serratula yamatsutana and Artemisia plant to endure drought) , saline meadow (L. chinensis with Chloris virgata and some plants to endure alkali and salt ), Herbage meadow (L. chinensis with some mesophytes ), and saline L. chinensis meadow. Along this geographical gradient (from east and west), annual precipitation varies from 579.2mm to 259.5mm, with falling extent about 320mm. Annual mean temperature ranges from 59℃ to 1.1℃, with falling extent about 4.8℃. The elevation increases from 160m to 1448m above sea level. However, L. chinensis maintains its dominance along such a steep environmental variables. Along the NECT, water use efficiency of L. chinensis was analyzed with δ¹³C, and the stoma density changes of L. chinensis leaves was observed and computed under microscope. The results showed that the ecological plasticity modulation of L. chinensis stoma density and water use efficiency were an important mechanism for its broad ecological adaptability. From east to west along NECT, the δ¹³C of the species varied from －27.49‰ to －23.57‰, and consisting with the reduction of annual precipitation, soil water and annual average temperature, but increased with the elevation of sampling sites. This suggested that the water use efficiency of L. chinensis increased from east to west along NECT. Along the same gradient, the stoma density increased from 969ind./mm² to 169.5ind./mm², indicating the water-use efficiency for the species was improved along the gradient. The linearity coefficient between the two parameters was significant (R² = 0.7338). The results of stepwise regression analysis showed that soil water was the first marked factor for determining stoma density, next was annual precipitation, this suggested that water factors were primary ecological factors influencing the stoma density of L. chinensis. Even though clear relationship between stoma density and δ¹³C was observed in this study, there are many complications remain unclear. For instance, the effects of annual mean temperature on stoma density was very complicated, in >4℃, the stoma density increased with annual mean temperature hoist, on the contrary, in < 4℃, the stoma density decreased with annual mean temperature hoist. In Suangliao and Balin County sampling plots, the particularity of soil (sand soil) and terrain (low land) reflected on the reallocation or fast loss of precipitation, and some sampling plots was different in human use mode and intensity. The findings in this study showed that responses of the stoma density and water-use efficiency for L. chinensis to environmental changes were very complex. They maybe an outcome by all environmental factors operated synthetically or by long term adaptation to different ecological environments for L. chinensis.