The Tertiary Red Clay in northern China is wind-blown origin, covering the period from ca. 5~8 to ca. 216Ma. In this study, we examine the clay mineral composition of the Red Clay and Quaternary loess-soils from the Zhaojiachuan section at Xifeng by using X-ray diffraction (XRD), Differential thermal analysis (DTA), Thermogravimetric Analysis (TG) and Phy-Chemical analysis. The results show that the clay minerals are dominated by illite, chlorite, smectite and kaolinite, an assemblage similar to the overlying Quaternary loess-paleosols. The concentrations of the major clayminerals calculated by comparingmajor peak areas, were estimated as follows: illite (53%~65%), smectite (9%~20%), chlorite (12%~20%), kaolinite (8%~13%). According to the spatial distributions of clayminera ls in modern soil, the claymineral assemblages indicate that it has been a semi-arid to semi-humid environment in the Chinese Loess Plateausince 6Ma. The high similarity of claymineral assemb lages between the Red Clay and Quaternary loess-soils also provides evidence for the wind-blown origin. In Quaternary loess deposits, although clayminerals mainly derived from source areas, the illite crystallinity of 110nm peak in Quaternary interglacial paleosol units is lower than in loess layers, indicating the post-depositiona l pedogenesis is the main cause for the change of the illite crystal structure, andmay be a good proxy for the summer monsoon evolution. The illite crystallinity is lower in the Red Clay than in the Quaternary loess, but being similar with the well-developed paleosols (such as S4 and S5), implying that the summermonsoon intensity is stronger in the Late Miocene-Pliocene than in the Quaternary.