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.