BaO thin films were grown epitaxially on the ultrasmooth SrTiO3 (001) substrate at temperatures ranging from 20 to 600 degrees C by laser molecular beam epitaxy. The film surface was characterized by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM). RHEED intensity oscillation at the specular beam spot could be clearly observed during the film growth in all the temperature range. The oscillation period corresponded well with the growth unit of biatomic (BaO)(2) layer, indicating a unit cell layer-by-layer manner of the growth. AFM analysis also proved the two-dimensional biatomic layer-by-layer growth.