The p-NiO thin film is prepared by radio frequency magnetron sputtering on the n-GaN/sapphire substrate to form p-NiO/n-GaN heterojunction diodes.The structural,optical and electrical properties of the p-NiO thin film are investigated.The results indicate that the NiO film has good crystal qualities and stable p-type conductivities.The current-voltage measurement of the p-NiO/n-GaN diode exhibits typical rectifying behaviour with a tu-on voltage of about 2.2 V.Under forward bias,a prominent ultraviolet emission centered at 375 nm is observed at room temperature.Furthermore,the mechanism of the light emission is discussed in terms of the band diagrams of the hetero junction in detail.