Electron mobility, minority‐carrier lifetime, and misfit‐dislocation density are measured along the layer thickness direction near the interfacial regions of GaAs crystals grown on GaAs and Ge substrates by vapor phase epitaxy. It is observed that the mobility and lifetime decrease, while the dislocation density increases exponentially, as the layer thickness is decreased. These phenomena are found to be consistently caused by out‐diffusion of interstitial Ga vacancies introduced at the epitaxial layer/substrate interface in the stage prior to growth or in the first stage of growth. The diffusion coefficient of the interstitial Ga vacancy (off‐centered Ga atom) is estimated to be 3.6×10−5 exp(−11800/T) cm2/sec based on the experimental results.