In effect on the surface kinetics modulation on the N-polarity GaN surface was studied by reflection high-energy electron diffraction (RHEED). It was found that there are two effects of In on the surface kinetics modulation. One is the surfactant effect as usually reported. Another one is the protection effect where In can suppress the evaporation of Ga atoms from the surface, and reduce the reaction between GaN and N to form Ga and N-2. We proposed a model to explain the phenomena.