To apply carbon isotope composition(? 13C) analyses of C4 plants to the quantitative reconstruction of paleoclimate, the functional mechanism linking plant ? 13C(? 13Cp) to the environment, which is based on the plants’ physiological characteristics and morphological adaptability, must be thoroughly understood. Foxtail millet(Setaria italic) and common millet(Panicum miliaceum), as C4 plants, are representative crops of the rain-fed agriculture present in northern China. Fossil millets are ideal for paleoclimatic studies because of the ease of acquisition and identification to the species level. Modern seeds of foxtail and common millet collected from different habitats of the Chinese Loess Plateau, and their carbon isotope compositions, were analyzed and correlated with environmental factors, such as latitude, altitude, temperature, precipitation, water availability, and relative humidity. The results showed that the ? 13 C of foxtail millet had a significantly negative correlation with latitude(R=?0.46), which may indicate the influence of light. The effect of light on the ? 13 C of foxtail millet accounted for only 21% of variability, while other climatic factors did not exert significant influences. Thus, the ? 13 C of foxtail millet was not suitable for extracting climatic information. The ? 13 C of common millet was significantly and positively correlated with precipitation during the growing period(R=0.75), explaining 56% of variability. The functional mechanisms analyzed, using the plants’ physiological characteristics and morphological adaptability, indicated that common millet can adapt to environmental changes because of stomatal sensitivity and some non-stomatal factors. Therefore, the ? 13 C of common millet can record precipitation during growth and is a promising factor for paleoclimatic reconstruction. To apply carbon isotope composition (? 13C) analyzes of C4 plants to the quantitative reconstruction of paleoclimate, the functional mechanism linking plant? 13C (? 13Cp) to the environment, which is based on the plants’ physiological characteristics and morphological adaptability, must be are understood. Foxtail millet (Setaria italic) and common millet (Panicum miliaceum), as C4 plants, are representative crops of the rain-fed agriculture present in northern China. Fossil millets are ideal for paleoclimatic studies because of the ease of acquisition and identification to the species level. Modern seeds of foxtail and common millet collected from different habitats of the Chinese Loess Plateau, and their carbon isotope compositions, were analyzed and correlated with environmental factors, such as latitude, altitude, temperature, precipitation, water availability, and relative humidity. The results showed that the? 13 C of foxtail millet had a significant negative correlation wi The effect of light on the? 13 C of foxtail millet accounted for only 21% of variability, while other climatic factors did not exert significant influences. Thus, the 13 C of common millet was significantly and positively correlated with precipitation during the growing period (R = 0.75), explaining 56% of variability. The functional mechanisms analyzed, using the plants’ physiological characteristics and morphological adaptability, indicated that common millet can adapt to environmental changes because of stomatal sensitivity and some non-stomatal factors. Thus, the? 13 C of common millet can record precipitation during growth and is a promising factor for paleoclimatic reconstruction.