Vacancy-type defects in GaN with different carbon concentrations ([C]=2×10¹⁶−1×10¹⁸cm⁻³) were probed using monoenergetic positron beams. 1.5-μm-thick GaN layers were grown on Si substrates by metalorganic vapor phase epitaxy. Measurements of Doppler broadening spectra and positron lifetimes indicated that the major defect species in the GaN layers was a Ga vacancy coupled with nitrogen vacancies. The positron trapping rate of the defects decreased with increasing [C], which was attributed to the downward shift of the Fermi level position due to C acceptor and the resultant change in the defect charge state (neutral to positive). Under illumination, the defect charge state was changed from positive to neutral, which was attributed to the trapping of the excited electrons by the defects (V⁺+e⁻→V⁰). The effect of illumination was suppressed for the GaN layer with [C]≥10¹⁸cm⁻³, which was ascribed to the excellent electron trapping property of C in GaN.