Abstract:The Red Earth Formation in the middle lower Yangtze River is an import ant pedo stratigraphical unit in southern China. This formation is among the b e st geological records of Quaternary environment that bear information of climate changes for the subtropical zone of China. The study on the origin of this form ation is the basis for further investigation of the paleoclimatic information. I n this study, grain size was analyzed on the samples from Jiujiang section at J i ujiang city of Jiangxi Province, and the result was compared with that of the ty pical Quaternary eolian deposit, as well as the lacustrine and fluvial sediments . Grain size distribution was determined with a Malvern Mastersizer 2000 partic l e analyzer with a precision less than 1%. The analysis on grain size indicates t hat the Xiashu loess and vermiculated red soil (VRS) in Jiujiang section consist of uniform fine particles. The particles display a bimodal distribution, with g rain size fraction of 5～50 μm dominantly, and > 63μm in a small part. The di stribution is essentially similar to that in the Xifeng loess samples, but diffe r from the multimodal patterns of the lacustrine and fluvial deposits. The Xiash u loess and VRS are also similar to the Xifeng loess samples in grain size para meter. The mean grain sizes of the Xiashu loess and VRS are about 64～68 (90～118 μm) and 71～72(68～73 μm) respectively, somewha t finer tha n thos e of the Xifeng loess samples. The Xiashu loess, VRS and Xifeng loess have a sma ller mean square deviation value than the fluvial samples, indicating a much bet ter sorting and relatively stable depositional dynamics. The Xiashu loess, VRS a nd Xifeng loess samples show a positive skewness, but the fluvial samples a nega tive one. The lacustrine samples also show a positive, but obviously larger skew nes s. In kurtosis value, the Xiashu loess and VRS samples are close to the Xifeng l oess, and the fluvial sediments are obviously smaller than the lacustrine sample s. All these indicate that the Red Earth Formation in Jiujiang area have the sam e origin as the Quaternary loess in northern China.