Plant carbon isotope composition (δ¹³C) comprehensively reflects the information of C and H₂O exchange in the process of plant photosynthesis, thus, in theory it could serve as a potential indicator of long-term water use efficiency (WUE) of plants. In addition, it could also be used to reveal a series of climatic and environmental information related to plant physiology and ecology. In this paper, through the systematical measurements of δ¹³C values of samples representing 28 families and 118 C₃ herb species along the isohyet of 400 mm in farming-pastoral zone in northern China, the responses of δ¹³C values and WUE for C₃ plants to temperature gradients were analyzed. This study shows that, along the isohyet of 400 mm, the δ¹³C values of C₃ plant species in farming-pastoral zone range between -31.5 ‰ and -23.0 ‰, with an average value of -27.7‰. The distribution range of plant δ¹³C values in this study is consistent with that of C₃ plants grown in arid and semi-arid area on the Loess Plateau obtained by previous study. Statistical analyses for the whole C₃ plants show that the δ¹³C values of C₃ plants and the mean annual temperature were significantly and positively correlated with an increase in δ¹³C of about 0.14 ‰ per 1.0℃ increase, and that the δ¹³C values of C₃ plants also significantly and positively relate to the summer temperature, with a increasing rate of 0.27‰ per 1.0℃ increase. The results indicate that plant WUE increases with rising air temperature. However, the phenomenon above is only a false appearance, the variations in δ¹³C values of C₃ herbaceous plants with temperature are actually controlled by the water stress induced by the lower soil relative humidity (or humidity index) which is closely related to temperature increment. The available water for plant species growing in this belt transect is a controlling factor of plant carbon isotope fractionation. In addition, this study shows that the δ¹³C values of 5 C₃ eurytopic plants also increase with increasing temperature, but both the slope variations and the average δ¹³C values among species are significantly different, which suggests that different species have different strategies to adapt to environmental changes. Also, this study shows that, for different life-span herbaceous plant, δ¹³C values and WUE denoted by carbon isotope ratio present the following patterns: the δ¹³C of biennial herbs is smaller than that of perennial herbs, and larger than that of annual herbs (This could be associated with the root distribution and water absorption ability of different life-span herbs), and the finding is consistent with the results reported by Ehleringer in desert regimes, while opposite to the results from humid climate zone. This indicates that the δ¹³C values and WUE of different life-span herbs are likely related to local moisture condition.