Abstract The influence of Si concentration on the shape transition of self-assemble d SiGe islands was investigated. SiGe islands with different Si concentrations were grown by UHV/CVD. The topography and size distribution of islands were cha racterized by atomic force microscopy. The results show that the critical volum e increases with Si concentration, at which the islands change from pyramids to domes. A modified m odel was established and used to explain the influence of Si concentration on the sha pe transition by introducing the revised strain energy term depending on Si conc entration. Domeshaped as well as pyramidshaped unimodal SiGe islands were gr own under suitable conditions. This research indicates that the shape and size o f the selfassembled ialnds can be controlled more accurately by adjusting Si c oncentration.