The formation of the Ni/Si interface was investigated by using molecular beam epitaxy to deposit Ni on hydrogen-terminated Si(lll) surfaces at room temperature. Reflection high-energy electron diffraction observations showed that a sample on which 0.8-nm-thick Ni had been deposited shows the same 1 x 1 streak pattern that the original hydrogen-terminated Si surface did. And X-ray photoelectron spectroscopy measurements of the Si 2 p core-level showed that all the hydrogen atoms terminating the original Si surface are still there after the Ni deposition. This indicated that the Ni atoms diffused beneath the Si surface without breaking the surface Si-Si bonds. It thus seems that hydrogen atoms terminating the Si surface dangling bonds suppress the silicide reaction at room temperature.