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Undoped GaAs epilayers are grown on heavily In‐doped dislocation‐free GaAs substrates by the organometallic vapor‐phase epitaxial method. Some phenomena related to the misfit dislocation generation caused by the lattice mismatch between epilayer and substrate are investigated as a function of epilayer thickness. As the epilayer thickness increases, the surface morphology changes from mirrorlike to a streaky or cross‐hatched pattern, and also the perpendicular lattice constant increases gradually from the lattice constant of coherently grown epilayers to that inherent in pure GaAs. These phenomena are related to the relaxation process from the coherently grown crystalline lattice to that inherent in GaAs by generation of misfit dislocations. This relaxation process is also ascertained by both the increase of the exciton peak energy implying the recovery of the biaxial tensile strain and the increase of etch pit density, revealing the generation of the misfit dislocations at the epilayer/substrate interface.