The effect of small addition of Sc on the microstructural evolution in Al-15Ag alloy was investigated by Monte Carlo simulation and transmission electron microscopy (TEM). Monte Carlo simulation results indicate that Ag clustering is suppressed by the addition of Sc during the initial ageing stage, and there is a little chemical repulsion between Ag atoms and Sc atoms. TEM observations show that the morphology of γ precipitation can be modified by the small addition of Sc, which does result in a refinement of γ precipitates in Al-15Ag-0.2Sc. The mechanism by which trace Sc affect the microstructure is to reduce the ledge density of γ precipitates.