Abstract: The mineral composition of cohesive soil has a significant impact on its engineering and mechanical properties. There are some sophisticated methods and technology to identify clay minerals. However, the quantitative measurement of clay minerals content in soil is still a question. The paper studies the chemical and mineralogical compositions of red clays in Wuming and Guilin by means of chemical analysis, X-ray diffraction test, Bogue method and K-value method. The results show that Kaolinite, gibbsite, goethite are found in both clays and quartz is additionally found in Guilin red clay. The contents of Kaolinite, gibbsite, goethite(2mm) and goethite(2mm) are respectively equal to 74.0%, 12.53%, 2.22% and 8.37%in Wuming red clay. The contents of Kaolinite, gibbsite, goethite and quartz are respectively 56.59%, 11.44%, 15.61% and 12.45%in Guilin red clay.