Abstract: Permeability and erosion experiments are carried out on Gaomiaozi (GMZ) bentonite specimens with the dry density of 1.70 g/cm³, where NaOH solution is employed to simulate hyper-alkaline pore water, which is likely be produced in highly active nuclear waste repositories. Surface properties alterations of the specimens observed with the scanning electron microscope (SEM) and the transmission electron microscope (TEM) are used to interpret the influence of hyper-alkaline solution on the swelling potential, permeability, porosity and composition of bentonite, and to assess the micro-mechanism of hyper-alkaline pore-water effect on the properties of bentonite. The results show that GMZ bentonite presents a flake layer structure, and that gels with the shape of wings are formed on the surface of montmorillonite during hydration. The dissolution of gels produced by hydration and the destruction of bentonite structure by erosion due to hyper-alkaline water are observed too. A positive relation between alterative degree and solution concentration is also observed. The surface erosion of bentonite specimens eroded by hyper-alkaline solutions is shown as the dissolution of montmorillonite, which is identical to the results obtained using X-ray diffraction (XRD) tests. Therefore, submitting the bentonite to long-term infiltration of hyper-alkaline pore-water may lead to dissolution of montmorillonite, destruction of bentonite structure, increase of porosity, decrease of swelling potential, and amplification of permeability as well, which generally weakens its sealing and buffering properties.