Mg ion-implantation was carried out into very pure GaAs made by molecular beam epitaxy, in which the dose of Mg atoms, [Mg] was varied from l×1015 to 1×1020 cm−3 . As a method of characterization, low temperature photoluminescence (PL) studies were carried out as a function of [Mg]. A new emission, denoted by [g-g] was observed between bound exciton emissions and the emission due to 1 and-to-acceptor pairs. It was found that [g-g] appears around [Mg]= 2×1016 cm−3 and becomes a dominant emission at [Mg] over l×1017 cm−3. A significant shift of [g-g] towards the lower energy side with increasing [Mg] was reproducibly obtained and its PL properties were carefully investigated in terms of annealing temperature, TA and excitation density. Results revealed that the implanted Mg atoms become optically active at TA over 600 [g-g] was suggested to be acceptor-acceptor pair, which is consistent with the theoretical explanation that its radiative mechanisms is ascribed to the pair between excited-state acceptors. The systematic energy shift inherent to [g-g] was attributed to the growing ionization energy of the pair with increasing pair distance.