Metal-semiconductor-metal structured GaN ultraviolet photodetectors are fabricated on sapphire substrates by metalorganic chemical vapor deposition. The properties of GaN photodetectors are improved through thermal annealing. With a 3 V bias,the very low dark current is about 200 pA,the maximum responsivity of 0.19 A/W is achieved at 362 nm,and the corresponding detectivity is 1.2×1011cm·Hz1/2/W. The physical mechanism about the effects of thermal annealing is studied,which is attributed to the introducing Au by the thermal annealing.