Photoluminescence (PL) and electrical properties of implanted manganese ions in GaAs are reported. Mn can be introduced into Ga site by ion implantation followed by annealing at a temperature above 600°C. The intensity of the Mn omission line increases between 600 and 800°C. The highest PL intensity and the maximum carrier concentration are obtained at the same dose, about 1×1015 cm-2, where the carrier concentration is almost the same at the maximum value that has been reported in the melt-grown or epitaxial GaAs. Anomalous diffusion of implanted Mn in GaAs annealed at 800°C is found from the measurement of the PL intensity profile. The diffusion of implanted Mn in GaAs is suppressed when an amorphous layer has been formed and the interstitial-substitutional diffusion model provides the best qualitative explanation for this suppression phenomenon.