The structural, electrical and optical properties of Mg-doped GaN epitaxial layers grown by metalorganic chemical vapor deposition were investigated in order to optimize the p-type conduction. A series of samples exhibiting different Mg concentrations were involved with this study, and post-growth thermal annealing at temperatures ranging from 650 degrees C to 900 degrees C were performed under flowing N-2. Under our growth conditions, these two factors, i.e. the doping level and annealing temperature, were found to strongly control the physical characteristics of the samples. The highest hole concentration, of about 7 x 10(17) cm was recorded for the specimen prepared with a Mg source flow rate of 0.3 mu mol/min(-1) and subsequently anneal at 900 degrees C without damaging the surface morphology. The corresponding as-grown wafer also exhibited the minimum X-ray rocking curve full width at half maximum in both the symmetric (0 0 2) and asymmetric (1 0 2) diffraction planes.