We have shown that UV-light irradiation during plasma process anomalously enhances in-diffusion of the defects deactivating the Si donor in n-GaN. The region where the donor was deactivated by the plasma-induced defects became several times deeper upon the superimposed UV-light irradiation than that in the sample just exposed to plasma emission. It was also found that external light with the below-band-gap energy did not affect the defect in-diffusion, and hence we can suggest that the electron-hole pair generation is essential for the enhancement of defect in-diffusion. (C) 2013 The Japan Society of Applied Physics