Atomic force microscopy was used to study the shape transition of SiGe islands on Si (100) by ultra-high vacuum chemical vapor deposition. The influence of Si concentration on shape transition of SiGe islands was investigated. The results show that the critical volume increases with Si concentration, at which the islands change from pyramids to domes. A modified model was developed to explain the influence of Si concentration on the shape transition. Unimodal dome-shaped and pyramid-shaped SiGe islands were grown under proper conditions, respectively. This work demonstrates that the shape and size of SiGe islands can be controlled accurately through adjusting Si concentration in islands. (C) 2003 Elsevier B.V. All rights reserved.