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For InAs/GaAs quantum dot system, the evolution of the wetting layer (WL) with the InAs deposition thickness has been studied by reflectance difference spectroscopy (RDS). Two transitions related to the heavy-and light-hole in the WL have been distinguished in RD spectra. Taking into account the strain and segregation effects, a model has been presented to deduce the InAs amount in the WL and the segregation coefficient of the indium atoms from the transition energies of heavy-and light-holes. The variation of the InAs amount in the WL and the segregation coefficient are found to rely closely on the growth modes. In addition, the huge dots also exhibits a strong effect on the evolution of the WL. The observed linear dependence of In segregation coefficient upon the InAs amount in the WL demonstrates that the segregation is enhanced by the strain in the WL.Keywords Quantum dots AE Wetting layer AE Reflectance difference spectroscopy AE Segregation PACS 78.67.Hc AE 68.65.Hb AE 73.21.Fg A strained epilayer with a thickness beyond a critical value will change from layer-by-layer growth (Frankvan der Merwe growth) to island growth (StranskiKrastanow growth) in order to relax strain energy. When covered by barrier materials, these self-assem-