The impurity effects on the creation of Ga vacancies in Si‐doped GaAs grown on a Be‐doped epilayer by molecular‐beam epitaxy were investigated through slow positron beam measurements. Doping of Si impurities enhances the creation of Ga vacancies in GaAs. The experimental results support the theoretical prediction of the creation of Ga vacancies in terms of the change in the Fermi‐level position by the Si doping into GaAs, and also suggest that a Si atom diffuses in GaAs as a neutral complex of SiGa‐VGarather than that of SiGa‐SiAs. The change in theSparameter distribution at the interface between Si and Be‐doped regions is explained by the Be carryforward phenomenon which occurs during the growth of Si‐doped GaAs on a Be‐doped epilayer.