We have been able to gain insights into ferroelectric thin films from measurements of the thickness-dependence of various characteristics of these films, such as coercive field (E-c), remnant polarization (P-r) and leakage current. Fully-strained SrRuO3 (SRO)/BaTiO3 (BTO)/SRO hetero-structures with ultra-thin BTO layers from 30 nm to 5 urn were deposited on SrTiO3 (001) substrates by pulsed laser deposition. Well-defined interfaces between the ferroelectric BTO layer and the electrode SRO layer were confirmed by a tunneling electron microscope image and by the thickness-independence of their coercive fields. The ferroelectric hysteresis loop was observed in the thinnest BTO layer (5 nm thickness). The decrease of ferroelectric polarization was observed as the BTO thickness decreased reduced, which agreed well with the theoretical prediction. Resistive switching behavior was not observed in the thinnest films, but was observed in thicker films after dielectric breakdown. (c) 2005 Elsevier B.V. All rights reserved.