Luminescence evolution of GaN irradiated by low energy electron beam before and after hydrogenation has been investigated by means of cathodoluminescence （CL）, in connection with the diffusion properties of hydrogen in GaN. It is found that under low energy electron beam irradiation, the band to band emission of GaN shows a decrease before hydrogenation, while it shows an initial increase and a subsequent decrease after hydrogenation, and the decrease after hydrogenation is relatively weak. Moreover, there is no luminescence recovery in 20 hours after the first irradiation after hydrogenation. The experimental results indicate a luminescence enhancement effect of hydrogen by passivating certain defects in GaN. However, such effect must be realized by overcoming high migration barrier in GaN. In the experiments, low energy electron beam supports enough energy to hydrogen to diffuse and passivate defects in GaN. These results show strongly the importance of diffusion of hydrogen in the passivation process in semiconductors.