The initial silicide formation during the deposition of Pd to room-temperature Si (111) substrate and the dependence of Pd2Si-Si orientation relationship on Pd film thickness have been studied by TEM. The results show that the only silicide phase formed is Pd2Si under annealing temperature from 170℃ to 600℃. A thin layer of Pd2Si has grown without annealing, even when the substrate is kept in room-temperature. The Pd film thinner than 100 nm leads to epitaxial Pd2Si layer on Si (111), while the Pd film of about 400 nm gives the [0001] Pd2Si fiber texture. Various kinds of defect with high density exist in the silicide film and the interface.