Abstract: In recent years, increasing interest has been focused on the Quaternary red clay (QRC) in the middle and lower reaches of the Yangtze River. The provenance of the QRC is a common concern. Grain size analysis, as a conventional technique, has been widely used to study the provenance and transport dynamics. But the pretreatment of samples are different for different researchers. In the QRC, there are many fine ferromagnetic nodules which can not be separated by hand. The existence of these tiny ferromagnetic nodules influences the analysis results seriously. This paper tries to find the influences of the nodules and the way to remove it. Our results showed that:(1) fine ferromagnetic nodules have a great influence on grain size analysis, especially they may increase the content of coarse components. And hydrochloric acid can effectively react with fine ferromagnetic nodules, making the grain size results closer to the true value. Therefore, for the red clay, which contains fine ferromagnetic nodules, the hydrochloric acid is necessary in the pretreatment of grain size analysis for the purpose of sedimentary dynamics analysis. (2) The hydrochloric acid has a great influence on the < 4μm group. It is not recommended in the pretreatment of grain size analysis for the purpose of weathering features analysis.