The clay minerals in two soil profiles of cinnamon soil in %iangshan district of Beijing were studied by chemical analysis, cation exchange capacity determination differential thermal analysis and %-ray diffraction. The results obtained are as follows: 1. The minerals in the clay fraction are composed of illite, strongly hydrated illite, illite-montmorillonite interstratified mineral, hydrobiotite and vermiculite. 2. The transformation sequences of clay minerals in cinnamon are supposed as follow.