Heteroepitaxial GaN film was grown on sapphire (0 0 0 1) substrate by hydride vapor-phase epitaxy and it was observed employing scanning electron microscopy and cathodoluminescence that there were microstructural inhomogeneous areas showing suppressed luminescence characteristics compared with the surrounding matrix. They were presumed to be induced by polarity-inverted domains identified on the basis of the chemical inertness and heavily doped characteristics manifested in cathodoluminescence spectra.