The structure of diamond/silicon interface, which was formed by the contact and the subsequent relaxation of the unreconstructed (001) surfaces of diamond film and silicon substrate, has been investigated by molecular dynamics simulation.The interaction among atoms of the silicon/carbon binary system was described by Tersoff many-body empirical potential. Before relaxation, the ratios of silicon atoms to carbon atoms along 110 and 110 directions are both 3∶2 and the ratio of the total number of interfacial silicon atoms to that of carbon is 9∶4. After relaxation, the matching of diamond and silicon lattices has been changed: along110 direction, a 3∶2 coincidence relation is roughly remained, but along 110 direction, a 1∶1 coincidence relation is approximately adopted. Accordingly, the ratio of the total number of interfacial silicon atoms to that of carbon is near 3∶2. The migration of partial silicon atoms, which were originally in the second layer, upward to the interface is responsible for such changes. The silicon lattice near the interface shows the tendency of disordering along 001 direction. The strong bonding between interfacial silicon and carbon atoms was found and Si-C bonds with an average length of 0.189 nm were formed. This study has confirmed that the main feature of lattice matching is the meeting of silicon atoms at and near the interface to the arrangement of interfacial carbon atoms.