The determination of plant carbon (C) isotopic concentration is related to the C assimilation and diffusion of CO₂ influenced by water stress. In addition, the determination of plant nitrogen (N) isotopic concentration is related to the availability of nutrients and water, and is indicative of N cycling on different spatial and temporal scales. The question arises as to whether a relationship exists in the processing of C and N by vegetation across various physical environments in temperate steppes as evidenced by the natural abundances of C and N in foliage (δ¹³C and δ¹⁵N). Given the strong precipitation and temperature variation, Inner Mongolia is an ideal region for this study. The regional patterns of foliar δ¹³C and δ¹⁵N along the Northeast China Transect (NECT) in Inner Mongolia steppe, and their relationship with environmental factors, which are mean annual precipitation (MAP) and mean annual temperature (MAT), have been studied. We collected 158 samples, which included 18 species of C₃ plants and 5 species of C₄ plants. The δ¹³C values of C₃ plants in this region range from -28.87‰ to -22.53‰, while those in C₄ vary from -14.06‰ to -11.64‰. The δ¹⁵N values of plants in this region range from -2.63‰ to 8.57‰ with a mean value of 2.13‰, and most values (80% of all data) are higher than 0‰. Our results show that δ¹³C values in C₃ plants decrease significantly with increasing MAP (R²=0.549, P < 0.001), such that the coefficient of δ¹³C-MAP is -1.16‰/100 mm. However, a positive linear relationship exists between the δ¹³C values of C₄ plants and MAP (R²= 0.188, P < 0.05). Foliar δ¹³C values of C₃ plants also decrease significantly with increasing MAT (R²=0.549, P < 0.05), such that the coefficient of δ¹³C-MAT is -0.14‰/1℃. However, MAT does not significantly affect the δ¹³C values of C₄ plants (R²=0.032, P=0.432). The δ¹⁵N values decrease significantly with increasing MAP, both for C₃ plants (R²=0.373, P < 0.001) and C₄ plants (R²=0.319, P < 0.01); i.e., plant species occupying a dry habitat has a higher δ¹⁵N value as compared to species growing in wet environments, irrespective of whether they are C₃ or C₄ plants. However, MAT does not significantly affect the δ¹⁵N values of C₃ (R²=0.373, P=0.053) and C₄ plants (R²=0.023, P=0.514). Therefore, our conclusions are that the dominant species in this region are C₃ plants, and the foliar δ¹³C of C₃ plants in this region is dominated by MAP and MAT, and that of C₄ plants is only affected by MAP. Furthermore, both for C₃ and C₄ plants, MAP is an important factor affecting the foliar δ¹⁵N, while MAT does not significantly affect the foliar δ¹⁵N values of plants in Inner Mongolia.