The influence of Si ion implantation in GaN and post implantation thermal annealing on yellow luminescence(YL) by using two types of GaN samples with strong YL and without YL were studied. As the thermal annealing temperatures increased, the YL intensity and the intensity ratio of YL to near band edge(BE) emission ( I Y? I BE ) for both types of implanted GaN samples enhanced. After Si ion implantation and post implantation thermal annealing, the YL of the GaN sample with strong YL decreased markedly, while that of the GaN sample without YL increased markedly. These experimental results show that the ion implantation together with appropriate post annealing can produce YL related deep acceptor defects, and can increase YL intensity, besides, the GaN surface can adsorb two kinds of point defects, one is caused from ion implantation, another is native and related to YL. This adsorption action of GaN surface becomes strong as ion implantation dose increases.