We have demonstrated formation of ultrathin Ni-silicide on SiO₂ by annealing a-Si/Ni/a-Si structures and have evaluated an impact of a-Si film thickness on their silicidation reaction. As a result, XPS analyses confirm that suppression of Ni oxidation due to reductive reaction caused by the top a-Si layer makes it possible to form the ultrathin Ni-silicide layer. Consequently, we successfully formed 2-nm-thick Ni-silicide layer by controlling the Ni oxidation.