Clay mineral composition and illite crystallinity (IC) value of the Quaternary Red Clay (QRC) in Xuanzhou District (XZ) and Langxi County (LX) of Xuancheng City, Anhui Province, Southeast China, were studied to explore relationship between the two and potential paleo-environmental implications of IC value for formation of the QRC. Clay fraction of the QRC was extracted and then treated with citrate-bicarbonate-dithionite (CBD) to remove free Fe. Mineral composition of the clay fraction was analyzed using the X-ray diffraction (XRD) method, and the width of half maximum height of d₀₀₁ (1.0 nm) peak of illite was measured to calculate IC value. Contents of macro elements of the QRC were analyzed by the X-ray fluorescence (XRF) method, and SiO sub>2/Al sub>2O sub>3 (Sa), SiO2/(Al sub>2O sub>3+Fe sub>2O sub>3) (Saf), Ba value as well as indicators of weathering degree of the QRC, such as CaO/TiO sub>2, MgO/TiO sub>2, K sub>2O/TiO2 and Na sub>2O/TiO sub>2 were calculated. Results show that the clay minerals in different layers of Profiles XZ and LX are quite similar in composition, despite some slight differences. The layer of Yellow-brown Earth (YBE) in the upper part of the profiles is identified as Xiashu Loess formed during the Last Glacial Period, and its clay minerals consist mainly of illite, kaolinite and 2:1 type vermiculite, but in the Uniform Red Clay (URC) and Reticulate Red Clay (RRC), the clay minerals are dominated with illite and kaolinite, and nil of vermiculite. However, in the lower part of the RRC, the peak of illite has become wide and flat, and illite-montmorillonite mixed-layer minerals are formed from weathered illite. IC values of the two profiles range between 0.4 and 0.7, suggesting that crystallization degree of the illite lingers between the high and moderate levels. IC value in the profiles varies sharply between layers. It is 0.482, 0.578 and 0.735 on average, respectively, in the YBE, URC and RRC of Profile XZ and 0.454, 0.628 and 0.717 on average, respectively, in the YBE, URC and RRC of Profile LX. The QRC in this study is significantly higher than the loess (0.3-0.4), paleosols (0.35-0.5) and Tertiary Red Clay (0.4-0.5) in the Chinese Loess Plateau, suggesting that the QRC in Southern China is highly weathered, and consequently low in illite crystallinity. In Profile XZ, IC value is negatively correlated with Ba, K sub>2O/TiO sub>2, Na sub>2O/TiO sub>2 and MgO/TiO sub>2 to a significant extent (p <0.05), and in Profile LX, IC is negatively correlated with Saf, Ba, K sub>2O/TiO sub>2 and Na sub>2O/TiO sub>2to a significant extent, too (p <0.05). The correlations between K sub>2O/TiO sub>2 and IC value in Profiles XZ and LX are the most significant, with correlation coefficient (r) being -0.818 and -0.929, respectively, which may be closely related to the high volume of K existing in the interlayer of illite, and K loss from weathering illite and decomposition of illite crystallines of the QRC. This finding fully demonstrates that IC value can be used as an effective indicator of weathering degree of the QRC in Southern China. In Profiles XZ and LX, IC value increases steadily, indicating that the crystallization degree declines from the YBE to URC and to RRC. Weathering and decomposition of illite in the RRC may also be related to long-term intensive groundwater activities during the formation of vermiculate. In the QRC profiles, the rising trend of IC value with soil depth coincides with the variation of weathering degree with soil depth. So like particle size of red soil and elemental geochemical indices, IC value of QRC possesses some similar paleoclimatic implications. Therefore, IC value of the QRC can be regarded as one of the effective paleoclimatic indicators, reflecting evolution of the paleoclimate during the period when QRC was forming.