Abstract: The ageing effects on the microstructure of GMZ01 bentonite with different water contents and dry densities are investigated. Firstly, GMZ01 bentonite powder with three different water contents is statically compacted to the designed dry densities. Then, the compacted samples under each initial condition are kept at constant volume and water content for various periods of time (1, 30 and 90 days). Afterwards, the aged samples are subjected to MIP and SEM tests respectively. The test results show that the inter-aggregate porosity decreases and the intra-aggregate porosity increases with the ageing time. The volume of the very thin pores not intruded by mercury also increases with the ageing time. The microstructure of the aged samples tends to be more homogeneous. These observations may be attributed to the smectite hydration induced by suction decrease of the compacted samples during the ageing process. With hydration, the water molecular layers between the interlayer spaces increase, and simultaneously, the large clay particles with many clay layers are split into smaller particles with fewer clay layers, creating a number of interparticle pores inside the inter-aggregates. Under the constant volume conditions, the inter-aggregate pores are gradually filled, resulting in the decrease of the volume of the inter-aggregate pores.