On the basis of a review of the research advances in the clay mineral composition of loess in China, the results of clay mineral composition, detected quantitatively by the method of X-ray diffraction including techniques of sample treatment and automatic data processing, of the Malan loess from 12 counties of northShaanxi and west Shanxi are reported in this paper. The quantitative analysis results indicate that the dominant clay mineral in the Malan loess is I/S mixed-layer mineral, and not illite as reported by former researchers. The illite in loess exists both in the form of single mineral and that of I/S mixed-layer mineral. It is discovered that the mineral composition of the Malan loess is similar in different zones, but the I/S mixed ratio is locally increased in Shenmu, Hequ and Pianguan counties. An observation by SEM indicates that the clay mineral particles do not exist in the form of single particles, either in the Malan loess or the Lishi loess, but appear as appendixes surrounding the debris surface, which can fully reveal the geochemical characteristics of the loess source area. Furthermore, as an important expansive clay mineral in loess, the I/S mixed-layer mineral can help us to understand the engineering properties of the Malan loess, such as collapse deformation and strength attenuation under the impact of water.