We have investigated the spatial evolution of (n x 3) surface reconstructed areas during the molecular beam epitaxial growth of InAs-GaAs(001) by using ab initio-based calculation and in situ observation, in order to understand the mechanism of consequent QD nucleation. Statistical analysis of (n x 3)-reconstructed morphology reveals that the fraction of (8 x 3)-reconstructed areas, as well as those of (4 x 3) and (6 x 3), appears and decreases in the later stage of the growth. This behaviour is consistent with the result of our ab initio-based calculation incorporating chemical potentials of source materials in the gas phase. In contrast, fragmented (2 x 3) areas remain throughout the InAs growth as potential QD nucleation sites. (C) 2017 Elsevier B.V. All rights reserved.