La0.9Sr0.1MnO3 (LSMO) ultrathin films with various thickness (in the range of 5-50 unit cells) are grown on (0 0 1) substrates of the single-crystal SrTi0.99Nb0.01O3 by laser molecular-beam epitaxy (laser-MBE), and the surface morphology of these films were measured by scanning tunneling microscopy (STM). STM images of LSMO ultrathin film surface reveal that surface morphology becomes more flat with increasing film thickness. This study highlights the important effect of strain caused by the lattice mismatch between substrates and ultrathin films. And the results should be useful to the investigations on growing manganite perovskite materials. (C) 2007 Elsevier B.V. All rights reserved.