Great attention has been received in recent years for the applications of visible-blind and solar-blind photodetectors, such as flame sensing, secure space-to-space communications, and missile plume detection. Due to the direct wide band gap, GaN is an excellent choice for optoelectronic devices in the visible and the ultraviolet (UV) portion of the spectrum. The p-i-n structure is an attractive candidate for an UV photodetector because of its high responsivity, low dark current and the facility in integration. The p-i-n structured GaN photodetectors have been fabricated on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) in this paper. Furthermore, the carrier density of p-type GaN layer can be enhanced by thermal treatment in N2 ambience, which results in reducing dark current. The dark current of the device is only about 65 pA at 1 V bias. The maximum responsivity of the device is 0.92 A/W at 361 nm under 1 V bias.