Abstract: This paper provides a reslut of macroscopic and microscopic study of a new type of expensive soil, the eluvial-colluvial clay on basalt, found in Central Nei Mongol.The formation environment of the eluvial-colluvial expensive soil abnormally developed in Central Nei Mongol is revealed and the formation mechanism of bed engineering properties is discussed from the composition, physical and chemical properties, and microstructure of the soil. Then an engineering evaluation of the expensive soil is made.The finding of this type of expensive soil in northern China leads to revise the knowledge on regulaities of formation and development of expensive soil in northern China.