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Uncertainties would exist in the relationship between delta C-13 values and environmental factors such as temperature, resulting in unreliable reconstruction of paleoclimates. It is therefore important to establish a rational relationship between plant delta C-13 and a proxy for paleoclimate reconstruction that can comprehensively reflect temperature and precipitation. By measuring the delta C-13 of a large number of C-3 herbaceous plants growing in different climate zones in northern China and collecting early reported delta C-13 values of C-3 herbs in this study area, the spatial features of delta C-13 values of C 3 herbs and their relationships with humidity index were analyzed. The delta C-13 values of C-3 herbaceous plants in northern China ranged from -29.9% to -25.4%, with the average value of -27.3%. The average delta C-13 value of C-3 herbaceous plants increased notably from the semihumid zone to the semiarid zone to the arid zone; the variation ranges of delta C-13 values of C-3 plants in those 3 climatic zones were -29.9% to -26.7% (semihumid area), -28.4% to -25.6% (semiarid area), and -28.0% to -25.4% (arid area). In the semiarid zone, the semihumid zone, and the whole northern area, delta C-13 values of C-3 herbs showed obvious linear negative correlation to humidity indexes (P < 0.05). With the increase of humidity indexes, the average delta C-13 value of C-3 herbaceous plants tended to decrease to different extents. In the arid zone, however, a linear positive correlation was found between them (P < 0.05). With every 0.1 increase in humidity index, the average delta C-13 value increased significantly by 1.3%. Temperature is the main reason for different C-13 fractionation abilities of C-3 herbs occurring in different sampling sites. The highly varying response of delta C-13 of C-3 herbaceous plants to humid index remindus us that delta(1)3C(Plant)-based paleoclimate reconstruction in northern China should be carried out according to the different climatic zones.