Abstract: In order to investigate the ionic solutions of sodium, potassium, calcium and magnesium for inhibition of hydrated swelling of montmorillonite natural settling and filtration tests are made with montmorillonite. Test result indicates that: the volume expansion of montmorillonite in ionic solutions of the above mentioned elements is Ca²⁺＞ Mg²⁺＞ Na⁺＞K⁺ in order of degree of expansion; a higest montnorillionite filtration rate can be expected when it is placed in potassium ionic solution; and the montmorillionite swelling inhibition property of potassium ion is far superior to the other 3 ions.