Grain size of eolian deposits from the Loess Plateau in China has been widely used to reconstruct the history of the East Asian winter monsoon. However, the grain size of bulk samples is only partially indicative to the strength of the winter monsoon because post-depositional weathering processes have significantly changed the grain size of original eolian particles. Here, non-weathered loess samples were separated into eight different particle fractions, and major chemical elements were determined in order to establish a geochemical indicator of original eolian grain size. The results show that SiO₂ and Al₂O₃ contents and the SiO₂/Al₂O₃ ratio in different fractions vary regularly with grain size, and that a good linear relation exists between the SiO₂/Al₂O₃ ratio and grain size for the fractions <50 μm. Because Al and Si are among the most stable elements and pedogenic processes in the Loess Plateau cannot affect the SiO₂/Al₂O₃ ratio, this index can be used to reflect the grain size of original eolian particles. Application of this index in the Weinan and Luochuan loess sections of the last climatic cycle shows that SiO₂/Al₂O₃ is in good agreement with median grain size (Md) in the loess units. On the contrary, SiO₂/Al₂O₃ has documented a series of fluctuations in the soil units that are not clearly indicated by the grain-size changes of bulk samples.