The effect of irradiation by positive oxygen ions on the crystallinity of barium titanate (BaTiO3) thin films was investigated. The oxygen ions were generated by an electron cyclotron resonance source (ECR-gun) and their energy was controlled by an accelerating voltage, which was applied to the ECR-gun, and a bias voltage, which was applied to the substrate. Barium titanate thin films were deposited on SrTiO3 (001) substrates by a molecular beam epitaxy method using ionized oxygen fluxes. Epitaxially grown BaTiO3 thin films were formed at 400 degrees C when the accelerating voltage and the bias voltage were 50 and -100 V, respectively. The accelerating voltage was necessary for selection of the charged species in the ECR plasma and the bias voltage gave direction to the species towards the substrate and accelerated them in order to supply sufficient energy to enhance the surface migration of atoms. It was revealed that ionized oxygen with low energy was effective in both oxidation in a high vacuum of about 10(-6) Torr and reducing the crystallization temperature.