In this paper, we report organic carbon isotopic characterizations from two loess sequences (JY and GL), spanning the last 20 ka, from the northwest Chinese Loess Plateau (CLP). The results indicate that the vegetation type is nearly pure C-3 plants in the studied region during the Holocene. In contrast to other reported loess sequences in the central-southeast CLP, the relative abundance of the C-4 plants decreases from southeast to northwest, with the vegetation types changing from pure C-3 to a C-4/C-3 mixture should near 36A degrees N in the Holocene. From the perspective of the modern temperature and precipitation distribution, the summer temperature has no obvious change at the same latitude, but there are differences in the summer precipitation, which exhibit an obvious increase from west to east. Further analysis indicated that the C-4 plant abundance decreases with the decreasing summer season precipitation from the southeast to the northwest CLP during the Holocene. We suggest that with the absence of favorable precipitation condition, increasing temperature and decreasing atmospheric CO2 concentration are insufficient to drive an expansion of the C-4 plants on the CLP in the Holocene. According to a Holocene precipitation reconstruction, a "threshold value" of summer precipitation existed, which mainly controlled the expansion of C-4 plants. Compared with the modern delta C-13(som) and climate data on the CLP, both the Holocene and the present "threshold value" are near the 360-mm summer precipitation line, although the present precipitation line turned slightly southwestward. Our results provide new insights for further research on the C-4/C-3 variations with precipitation and the relationship to global C-4/C-3 change.