GaN ultraviolet (UV) detectors were fabricated on a freestanding GaN substrate with low dislocation density. The resulting dark current density was below 1 nA/cm-2 at -8 V reverse bias, which was about 3 orders of magnitude lower than that of a similar detector made on a sapphire substrate. Moreover, the ideality factor was nearer to unity than the device on a sapphire substrate. In addition, by comparing the GaN-based device to a commonly used Si photodetector, we found that the GaN device had a lower signal-to-noise ratio and greater temperature stability. Therefore, we found a drastic reduction of dark current by using GaN freestanding substrates and so the GaN substrate produced a more effective detector than the sapphire substrate.