Heavily Zn‐doped epitaxial layers have been grown on In‐doped GaAs substrates by organometallic vapor phase epitaxy in order to investigate the strain relaxation mechanism and hence the nucleation and propagation of misfit dislocations. Samples doped with Zn higher than 3×1020cm−3still show a perfectly strained state beyond the coherent length obtained for undoped GaAs epilayers. Hence, epilayers having a higher Zn concentration seem to have a higher activation barrier for nucleation and propagation of misfit dislocations. Furthermore, for the Zn‐doped/undoped/substrate(In‐doped) double‐epilayer structure, the elastic strain in the heavily Zn‐doped layer is found to be unrelaxed, regardless of the increase in the thickness of the Zn‐doped upper layer. This result suggests dislocation generation at the growing surface and glide along the {111} slip planes into the heterointerface.