beta-Ga2O3 thin films were grown on (100)-oriented beta-Ga2O3 single crystal substrates by plasma-assisted molecular beam epitaxy. At the growth temperature over 800 degrees C and the Ga beam equivalent pressure of 1.1 x 10(-7) Torr, the grown surfaces exhibited clear step and terrace structures and the root-mean-square roughness of 0.5 nm in atomic force microscopy. The successful step-flow growth is indebted to the strong cleavableness of (100) planes of beta-Ga2O3. The off-axis direction, determined by the initial substrate polishing stage, significantly influences on the surface morphology, and it should be along [001] or [00-1] directions in order for smooth surfaces. (C) 2007 Elsevier B.V. All rights reserved.