Clay mineralogical composition of a Yellow-brown soil of China was examined. Common to the A and C horizons, micas and their transformation products dominated the clay fraction. The transformation products of micas were, however, different between the fine (<0.2μm) and coarse (0.2-2μm) clay fractions. Irregularly interstratified mica/smectite or mica/vermiculite/smectite of the smectite-rich phase was found in the fine clay fraction, while smectite and regularly interstratified mica/smectite were detected in the coarse clay fraction. The endo-type and exo-type K-replacement models were proposed to explain the different transformation products between the two clay fractions.