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Water availability is the most influential factor affecting plant carbon (delta C-13) and nitrogen (delta N-15) isotope composition in arid and semi-arid environments. However, there are potential differences among locations and/or species in the sensitivity of plant delta C-13 and delta N-15 to variation in precipitation, which are important for using stable isotope signatures to extract paleo-vegetation and paleo-climate information. We measured delta C-13 and delta N-15 of plant and soil organic matter (SOM) samples collected from 64 locations across a precipitation gradient with an isotherm in northern China. delta C-13 and delta N-15 for both C-3 and C-4 plants decreased significantly with increasing mean annual precipitation (MAP). The sensitivity of delta C-13 to MAP in C-3 plants (-0.6 +/- 0.07 parts per thousand/100 mm) was twice as high as that in C-4 plants (-0.3 +/- 0.08 parts per thousand/100 mm). Species differences in the sensitivity of plant delta C-13 and delta N-15 to MAP were not observed among three main dominant plants. SOM became depleted in C-13 with increasing MAP, while no significant correlations existed between delta N-15 of SOM and MAP. We conclude that water availability is the primary environmental factor controlling the variability of plant delta C-13 and delta N-15 and soil delta C-13 in the studied arid and semi-arid regions. Carbon isotope composition is useful for tracing environmental precipitation changes. Plant nitrogen isotope composition can reflect relative openness of ecosystem nitrogen cycling.