Effects of post-growth annealing under dissimilar ambient fluxes on InAs islands grown on GaAs(001) substrates by low pressure metal-organic vapor phase epitaxy (LP-MOVPE) have been investigated. It is found that InAs islands annealed under different ambient fluxes exhibit different surface morphology evolution. Islands annealed under DMHy flux exhibit dissolving of the 3D island morphology. In contrast, upon annealing under TMSb flux, the 3D island morphology is enhanced with increased island density. The result suggests that we can fine tune the size and density of InAs islands by post-growth annealing under different ambient fluxes and this method is expected to be applicable to the growth of QDs of other material systems. Our findings are discussed in terms of the existence of critical nucleus in heterogeneous growth processes. (c) 2005 Elsevier B.V. All rights reserved.