The hydroxy-aluminum/polyaluminum modified clay and the clay only samples were examined by X-ray diffraction (XRD), unconconsolidated undrained (UU) triaxial strength test, unconfined compressive strength testing techniques, swelling potential tests and so on. The results indicated that the hydrophilic nature and swellingshrinkage property were changed drastically, that the free swelling ratio (Fs) could be reduced sharply, and that the particle size gradation, strength characteristics of the modified montmorillonite clay were improved greatly. The clay content decreased dramatically, while the silt content increased. This study suggests that the hydroxy-aluminum/polyaluminum can influence the mechanical and physical properties of montmorillonite clay by physicochemical interaction.