Abstract: Montmorillonite is one of the most common minerals, which has such strong hydrophilicity that it has a significant influence on the properties of clayey soils. The influence of montmorillonite on water stability and dispersivity of soil is studied by mixing different kinds and contents of montmorillonite using sedimentation tests, pinhole tests and crumb tests. The results show that the dispersivity of soils increases and the stabilization decreases with the increasing content of Na-montmorillonite, but the increasing content of Ca-montmorillonite holds the opposite properties. The researches indicate that the sodium ion is an essential factor of the dispersivity of clayey soils, and that different kinds of montmorillonite have different influences on the dispersivity. Na-montmorillonite can promote the dispersivity of soils, while Ca-montmorillonite can restrain the dispersivity of soils.