Abstract: The ageing effects on the swelling properties 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 samples with two dry densities. Then, the compacted samples with different initial conditions are kept for various periods of time (0, 1, 7, 15, 30 and 90 days) under constant volume and water content conditions. Afterwards, the aged samples are subjected to swelling pressure tests using the swelling apparatus. At the same time, the SEM tests are conducted on some samples after experiencing different ageing time. The test results show that the swelling pressure of GMZ01 bentonite decreases with the ageing time. The pressure decreases fast at the early days of ageing and then turns to stabilize after 30 days of ageing. The ageing effects on the swelling pressure of GMZ01 bentonite depend on the initial conditions of the samples. The higher the initial water content and dry density, the stronger the ageing effects. The SEM test results indicate that the smectites hydration occurs in the 90-day aged samples. Aggregates decompose, particles bond with each other and the void distribution seems to be more homogenous, leading to a matrix type macrostructure. The smectites hydration induced by water redistribution between different microstructure levels is the main mechanism for the decrease of the swelling pressure of GMZ01 bentonite.