Zinc oxide quantum dot (QD) structures were grown on Si(0 0 1) substrates by reactive electron beam evaporation at low substrate temperature. The samples were then annealed at various temperatures. Photoluminescence excitation characterization revealed the red shifts of the optical absorption peaks related to the QD transitions after the annealing of the samples, i.e. 340 nm for the as-grown QD sample, and 363 nm for the annealed sample. Effective mass approximation is used to interpret the observed optical transitions of the ZnO quantum dots. The quantum clot sizes grown by this method are estimated to be between 2.6 and 3.0 nm in diameters. Calculation also shows that the quantum-confinement effect of the ZnO QDs becomes very weak when the size of ZnO quantum dots is larger than 8.0 nm. (C) 2002 Elsevier Science B.V. All rights reserved.