Thickness dependence of ferroelectric and structural properties of BaTiO3 thin films were investigated. Stoichiometric BaTiO3 thin films were prepared by off-axis rf magnetron sputtering on polycrystalline Pt substrates at 700°C. Film thickness range was 2,100–20,000Å. Room temperature permittivity, frequency dependence of permittivity, and D-E hysteresis loops were measured and lattice parameters were determined as a function of the film thickness. It has been found that these properties had the strong dependence on film thickness, which was mainly due to grain sizes of BaTiO3 thin films. The main cause of thickness dependence of dielectric properties was thought to be crystallinity and stresses of thin films which is resulted from changes in grain sizes.