The effect of Ni thin film on the reflectivity of the indium tin oxide (ITO)/Ag mirror of GaN light-emitting diodes after annealing at 200°C was investigated. Samples designated as "ITO/Ag" were ITO/Ag films without a Ni layer. " ITONiAg " and " ITOAgNi " were ITO/Ag films with a Ni layer either between the ITO and Ag films, or on the back side of Ag film. " ITONiAgNi " were ITO/Ag films with Ni layers on both sides of Ag film. It was found that annealed ITONiAgNi had the highest reflectivity (96.3%) because two Ni layers can retard the agglomeration of Ag and the in-diffusion of Ag into ITO.