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. Domeshaped as well as pyramidshaped unimodal SiGe islands were gr own under suitable conditions. This research indicates that the shape and size o f the selfassembled ialnds can be controlled more accurately by adjusting Si c oncentration.