Abstract: According to the theory of the interlamellar hydration microscopic structure of montmorillonite proposed by Forslind, a formula for calculating the water content of montmorillonite is established after its hydration. When the montmorillonite absorbs only monolayer of water molecules, the water content is considered as the residual water content. The influences of the initial dry density and temperature on the residual water content of bentonite are investigated by using measured soil-water characteristic curves of four different kinds of bentonites. The results show that the residual water content of bentonite is proportional to the specific surface area of bentonite, the thickness and the density of hydrated layer of water molecules, regardless of the initial density and temperature. The comparison between the measured and predicted residual water contents shows that the proposed formula is reasonable.