Si-doped GaN with a carrier concentration of 6 x 10(17) cm(-3) was grown on c-plane sapphire substrates by molecular beam epitaxy (MBE equipped with a nitrogen RF plasma sourer. The thermal annealing process was performed in a nitrogen atmosphere with a tungsten halogen lamp as the thermal source. Double crystal X-ray diffraction (DCXRD) and 77 K photoluminescence (PL) have been employed to investigate the effects of thermal annealing on Si-doped GaN films. Both annealing temperature and annealing time have been varied to investigate the influence of different annealing processes on the sample. The DCXRD results show that the dislocation density in the sample decreases after thermal annealing. 77 It PL of GaN annealed longer than 300 s at 950 degreesC shows a new peak at 378 nm. The integrated intensity of the peak increases dramatically with increasing annealing time. The excitation power dependence of the 378 nm peak indicates that the emission was associated with donor-acceptor pair (DAP) transitions, and a similar peak has been observed in highly Si-doped GaN with a carrier concentration of 9 x 10(19) cm(-3). We attribute the acceptor in DAP to Si-N.