The impact of dimer structures forming at the surface on the internal atoms of the Si thin film was examined by using two types of models: silicon-on-insulator (SOI) and plate. In SOI models, a dimer formation was modeled at one side of the Si thin film. Plate models had two dimers at each surface, which had been considered as a Si bulk model in previous studies. First principles calculation showed that the deviations of Si atoms from first to fourth layers and total energies of SOI models did not differ remarkably from those of plate models. The internal atoms deeper than the fifth layer showed no deviation in some SOI models and had nonzero deviation in the other SOI models. All SOI and plate models showed the lower self-energy of a Si atom than that in the Si bulk. The layer-to-layer distance of internal atoms in the film became longer than that of atoms in the Si bulk. These results indicated that (i) Si films with dimer surfaces are relaxed in deviations of the whole film and (ii) the plate models correspond not to Si bulk but to silicon-on-nothing structures or Si thin film.