Development of wide bandgap GaN radiation detectors is reported. Current mode GaN detectors with diameter of 20-50mm were developed by using large area semi-insulting GaN single crystal as its sensitive medium. The characteristics of the detector were tested such as the I-V curve, the responsibility and sensitivity to γ-rays, the charge collection efficiency, and the time response to pulsed X-rays. The test results showed that a low resistance ohmic contact formed between the crystal and the metal electrode. The dark current was lower than 400pA at voltage of 600V, the charge collection efficiency was greater than 40%, and the time response to pulsed X-rays was at nanosecond order. We also develop GaN detectors with PIN structure, the dark current was lower than 10nA at voltage of 20V, difference about the detector performance before and after 14MeV neutron radiation was studied, the results show the neutron radiation influence the carrier transport characteristics evidently under low bias. We also study radiation damage mechanics of GaN material under proton and neutron irradiation of 1015cm-2 fluence, results show that GaN material has high resistance to proton radiation, proton radiation generates Ga vacancies or complex defects related to Ga vacancy in material which lead to yellow luminescence and blue luminescence change. HVPE has high resistance to neutron radiation, neutron radiation generates VGa defects which combine ON to form stable complex VGa- ON, leading to the increase of yellow luminescence.