High-quality BaTiO3 films with thicknesses ranging from 2.9 to 175 nm were grown epitaxially on SrRuO3-covered (001)-oriented SrTiO3 substrates by high-pressure sputtering. The crystal structure of these films was studied by conventional and synchrotron x-ray diffraction. The in-plane and out-of-plane lattice parameters were determined as a function of film thickness by x-ray reciprocal space mapping around the asymmetric (103) Bragg reflection. BaTiO3 films were found to be fully strained by the SrTiO3 substrate up to a thickness of about 30 nm. Ferroelectric capacitors were then fabricated by depositing SrRuO3 top electrodes, and the polarization-voltage hysteresis loops were recorded at the frequencies 1–30 kHz. The observed thickness effect on the lattice parameters and polarization in BaTiO3 films was analyzed in the light of strain and depolarizing-field effects using the nonlinear thermodynamics theory. The theoretical predictions are in reasonable agreement with the measured thickness dependences, although the maximum experimental values of the spontaneous polarization and the out-of-plane lattice parameter exceed the theoretical estimates (43 μC/cm2 vs 35- 2002;μC/cm2 and 4.166 Å vs 4.143 Å). Possible origins of the revealed discrepancy between theory and experiment are discussed.