The study of the variations of C3/C4 relative abundance in terrestrial ecosystem since the Late Miocene has been the “hotspot” of international community of geosciences during the past decades. In the Chinese Loess Plateau, several profiles have been studied to get the knowledge of past C3/C4 variations since the Late Miocene in this area. However, stable carbon isotopic data of pedogenic carbonate from this area have many inconsistencies in different aspects, such as: the beginning time of C4 expansion, the process of C4 expansion, the consistency of regional and global C4 expansion, and spatial gradient of C4 relative abundance and the main driven factor of C4 expansion in this area. This may reveal the internal unreliability of carbon isotopic composition of pedogenic carbonate as an indicator of past C3/C4 variations in this area. At the mean time, carbon isotopes of animal fossil from Linxia Basin of this area are remarkably different with carbon isotopes of pedogenic carbonate, which indicated that C4 plants became a significant fraction of local ecosystemm, which only existed in the Quaternary. Considering the reliability and timescale resolution of proxy indicator, we suggest that studies on organic matters should be reinforced in ongoing works in the future, in order to make the fact and driven mechanics of terrestrial C3/C4variations since the Late Miocene in the Chinese Loess Plateau clear.