In a temperature range of 750-1050 degreesC rapid thermal annealing was carried out to activate Mg impurities in GaN epilayers grown on sapphire by metalorganic chemical vapor deposition. Activation increased with temperature until near 950 degreesC, and then decreased very slowly with further increasing temperatures. Hall mobility decreased (from 16.5 cm(2)/Vs at 750 degreesC) with increasing annealing temperature until near 1000 degreesC (12.8cm(2)/Vs). The temperature-dependent Hall effects demonstrated very low compensation, around 1%, for all the samples. The low compensation and high mobility may be due to post-growth cooling in nitrogen atmosphere.