A high-resolution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyses. The soil organic carbon isotope (δ13Corg) varied be- tween ?22.6‰ and ?27.5‰ during the last glacial at the section. During the last interstadial, the δ 13Corg values were more negative than those in both early and late periods of the last glacial by 4‰. The iso- topic composition indicates a coupled response of the pure C3 plants to the temperature, precipitation and the concentration of atmospheric CO2. Decrease in temperature and the atmospheric CO2 concentration from the last interstadial to Last Glaicial Maximum (LGM) caused the organic carbon isotopes to be- come positive by 1.5‰―2.0‰. The amplitude of 4‰ in the δ 13Corg variation during the last glacial should be mainly caused by the precipitation change. Therefore, the δ 13Corg variations of the Yuanbao Section during the last glacial period documented the large-amplitude fluctuation of the monsoon precipita- tion, which is estimated to be 250―310 mm more during the last interstadial than that in the LGM, and 100 mm more than that during early last glacial. The rapid changes of the monsoon precipitation on mil-lennial scale during the last glacial have also been recorded in the isotopic variations in Yuanbao loess section. As the isotopic composition varies compli- catedly as shown in the loess-paleosol sequence, it cannot be simply attributed to the abundances of C3 and C4 plants or be used as an indicator of the summer monsoon variations. A high-resolution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyzes. The soil organic carbon isotope (δ13Corg) varied be-tween? 22.6 ‰ and? 27.5 ‰ during the last glacial at the section During the last interstadial, the δ 13Corg values ​​were more negative than those in both early and late periods of the last glacial by 4 ‰. The iso-topic composition indicates a coupled response of the pure C3 plants to the temperature, precipitation and the concentration of atmospheric CO2. Decrease in temperature and the atmospheric CO2 concentration from the last interstadial to Last Glaicial Maximum (LGM) caused the organic carbon isotopes to be positive by 1.5 ‰ -2.0 ‰. The amplitude of 4 ‰ in the δ 13Corg variation during the last glacial should be mainly caused by the precipitation change. Thus, the δ 13Corg variations of the Yuanbao Section during the last glacial period documented the large-amplitude flu ctuation of the monsoon precipita- tion, which is estimated to be 250-310 mm more during the last interstadial than that in the LGM, and 100 mm more than that during early last glacial. The rapid changes of the monsoon precipitation on mil-lennial scale of the last glacial have also been recorded in the isotopic variations in Yuanbao loess section. As the isotopic composition has compli- cated as shown in the loess-paleosol sequence, it can not be simply attributed to the abundances of C3 and C4 plants or be used as an indicator of the summer monsoon variations.