Raman‐spectra measurements were made on semi‐insulating (SI) GaAs heat‐treated in an atmosphere of H2or Ar to study the effect of structural changes or strains on a thermal conversion of Si GaAs. Correlations between Raman spectra and electrical changes of SI GaAs indicate that strains in the interface region between SI GaAs and a dielectric film is, at least, one of the origins of the thermal conversion, while the structural changes accompanying the generation of the TO‐phonon line has no effect on the thermal conversion.