The microstructure evolution, atomic ratio (ρ) of Mg and Si in different precipitates, and precipitation strengthening effects during the ageing process were investigated by HRTEM, atom probe tomography (APT) and hardness testing in LT24 aluminum alloy used for nuclear fuel cladding alternative materials. The results show that the early stage of ageing at 180 ℃ led to a significantly increasing of hardness and the formation of high density of solute clusters and Guinier-Preston (GP) zones in the alloy. The alloy reaches peak hardness after ageing at 180 ℃ for 4 h due to a significant increasing density of the β" precipitates. After the peak hardness, a hardness plateau is maintained for longer ageing time, because of the β" precipitate is still the main strengthening phase in the specimens. The precipitates grow larger and the ρ increases with the increasing of ageing time. The ρ in β" needles changes from 1.23 to 1.35. β" needles are the main precipitation strengthening phase of the alloy. The precipitation sequence during the early ageing treatment in alloy can be described as follows: supersaturated solid solution →solute atom clusters→solute atom clusters+GP zone→solute atom clusters+GP zone+β".