Radiocarbon and OSL dating are the two main approaches used to date aeolian deposits in arid central Asia (ACA). Previous results showed that, compared with OSL dating, the C-14 ages of loess deposits in ACA are underestimated when the loess age is beyond similar to 25 ka. However, the cause of this radiocarbon age underestimation has not been adequately addressed. In this study, we measured the C-14 ages of different dating materials, including pyrolysis volatile (Py-V), pyrolysis residue (Py-R), humin, and total organic carbon fractions from two loess sections in ACA (the Hoalin section in the western Pamir Mountains, and the TLD16 section in the northern Tianshan Mountains). The results show that different dating material yielded different C-14 ages for all the loess samples, and all the radiocarbon ages obtained are younger than their corresponding OSL ages. Based on these results, together with a regional comparison of 78 C-14 ages from ACA with their corresponding OSL ages, we propose that the C-14 age underestimation is mainly due to the contamination by the deep penetration of the extended root systems of the plants that grow in arid areas, whereas this effect is less significant in the summer monsoon-dominated Chinese Loess Plateau.