The authors report on the electrical properties of free-standing GaN substrates, using one typical set of low crystalline quality (type I) and high crystalline quality (type II), fabricated by hydride vapor-phase epitaxy and self-separation technique. Type I has a large electron concentration of ~10 18 cm −3 and a small electron mobility of ~85 cm 2 /Vsec while type II has a small electron concentration of ~5 × 10 15 cm −3 and a large electron mobility of ~400 cm 2 /Vsec. In type I, the electron concentration decreases with the decrease of temperature until a critical temperature and then increases at lower temperatures while in type II, the electron concentration continuously decreases with the decrease of temperature. These different electrical properties are ascribed to the difference in the structural defect densities of the two samples. These indicate that the electron-transport mechanism of the defective free-standing GaN substrates is dominated by the two-band conduction model. These results are consistent with the temperature dependence of the electron mobility and with the structural properties observed by using high-resolution X-ray diffraction and atomic-force microscopy.