Abstract: Clay minerals in the clay beds near the P/T boundary at Huaying Mountain, eastern Sichuan, were analyzed by X-ray diffraction analysis. The results show that the clay minerals are composed predominantly of ordered mixed-layer illite/montmorillonite, and that one sample possibly contains small amount of random mixed-layer chlorite/montmorillonite. Generally, the content of illite layers in the ordered mixed-layer illite/montmorillonite of the clay beds near the boundary shows a trend of decreasing from the base upward, with an average content of 83%, and the maximum paleotemperatures which they underwent ranged from 125 to 150℃. The composition and characteristics of these clay minerals are in good agreement with those at the same horizon in other areas of South China, showing that the clay minerals in the clay beds have the same origin and isochronous significance. Their differences (e.g. in the presence and content of kaolinite) in different areas may be related to whether an unconformity or hiatus exists at the P/T boundary; whereas the ordered mixed-layer illite/montmorillonite ratio is related to the burial history or depositional to post-depositional fluid medium.