【Objective】It is well known that soil clay minerals play an important role in sustaining soil fertility, and composition of soil clay minerals vary with vegetation and management systems and land use patterns. A field survey of the piedmont plain of Mountain Tai was carried out on variation of clay mineral composition in the aquic brown soil with land use, i.e. wheat-maize rotation, poplar plantation and barren land, its affecting factors and its relationship with soil fertility indices.【Method】Based on the field survey, soil samples were collected from the fields different in land use for extraction of < 2 μm soil clay particle with pipette method and analysis of composition of the extracted clay and relevant indices with the X-ray diffraction (XRD) method. XRD was applied to the clay samples for XRD patterns after the samples were treated separately with (a) the magnesium saturation and air-drying method, (b) magnesium saturation followed glycerol salvation, or (c) potassium saturation and heating at 300oC or 500oC method.【Result】Qualitative analysis of the experimental XRD patterns reveals that the soils, regardless of land use patterns, had illite as dominant clay mineral, which consisted of well crystallized illite (WCI) and poorly crystallized illite (PCI), vermiculite and kaolinite in the next, and mixed-layer minerals the least. Quantitative information of clay mineral fraction derived from the fitting procedure for the different sub-fractions allowed for determination of complex mineralogy of the < 2 μm clay fraction in the soil samples. Results show that the soil samples varied in percentage of clay mineral fractions with land use. The barren land was the highest in illite in the topsoil, reaching 75.8% and being, 26.4%, 23.6% than higher the agricultural soil and plantation respectively. The content of illite was significantly lower in the subsoil than the topsoil, especially for barren land. PCI varied similarly to illite in content. WCI content reflected weathering degree of the soil, hence it increased successively barren land to, plantation and agricultural soil. Vermiculite content in the topsoil was not significantly different between the three different land. However, it varied more remarkably in the subsoil than in the topsoil and in order of as barren land > agricultural soil > plantation. Kaolinite content was similar to vermiculite content in variation in the soils. Barren land soil was lower than the other two in soil organic carbon content and content of inorganic-organic complexes, too.【Conclusion】The content of illite in the soil negatively related to the content of vermiculite, but positively related to soil available potassium, which is attributed to the transformation between illite and vermiculite. The content of soil organic carbon and the formation of inorganic-organic complexes with sodium dispersion grows in agricultural soil as affected by tillage, fertilization, and so on, which inhabit movement of the clay minerals in the soil layer, enhance weathering of the soil minerals, and increase availability of the potassium embedded in illite. Nowadays more and more barren land is reclaimed into plantation and meadowland, thus increasing vegetation coverage and hence soil organic carbon content, and controlling erosion of the land.