Abstract: Data for geographic location, climatic conditions, and carbon isotope values of 325 C₃ herbaceous plant samples were obtained at 47 sampling sites through systematic investigation of a wide variety of natural habitats and δ¹³C data from the published literature of C₃ herb species in northern China. We calculated the humidity indices for different climatic areas in northern China and compared the spatial characteristics of δ¹³C composition and carbon isotope values for C₃ herbaceous plants with climatic environment factors (especially humidity index). The δ¹³C values for C₃ plant species in northern China as a whole ranged from –29.9‰ to –25.4‰, with a mean of –27.3‰. From the sub-humid to semi-arid and arid areas, the mean δ¹³C values of C₃ plants increased with decreasing mean annual precipitation. The variation in δ¹³C values differed among the 3 climatic areas in northern China, namely –29.9‰ to –26.7‰ (sub-humid area), –28.4‰ to –25.6‰ (semi-arid area) and –28.0‰ to –25.4‰ (arid area). Single-element regression analysis revealed a difference in relationship between δ¹³C values of C₃ herbaceous plants and humidity index among climatic areas. The δ¹³C values of sampling sites in semi-arid area, semi-humid area and the whole northern China all were negatively related to humidity index (P<0.05), and the mean δ¹³C values of plants decreased with increasing humidity index. In contrast, δ¹³C values in the northern arid area was positively related with humidity index (P<0.05). A 0.1 increase in humidity index in the arid area would produce a 1.3‰ increase in δ¹³C, and annual mean temperature has an important role in differences in humidity index and isotope fractionation among sampling points in the arid area.