We report on the critical thickness for InAs quantum dot (QD) formation on (311)B InP substrates. Firstly, critical thicknesses for InAs QD formation on InP surfaces have been measured by reflection high-energy electron diffraction. Large change of the critical thickness has been observed as a function of substrate temperature. We assume that is related to large As/P exchange on InP surface which leads to the formation of extra InAs on surface. Then, change of critical thickness during QD stacking has been investigated. When capping layers were grown continuously a large decrease of the critical thickness was observed as a function of the number of QD layers. In contrast, when capping layers were grown in two steps (double cap procedure) a nearly constant critical thickness was measured. We propose an explanation based on stress-driven mass transport and As/P exchange on InP surface to interpret such results. (C) 2009 Elsevier B.V. All rights reserved.