The onset of lattice strain relaxation against the number of bilayers (N) in an epitaxial BaTiO3 (3 nm)/LaNiO3 (3 nm) superlattice grown on a SrTiO3 (0 0 1) substrate was characterized in situ by X-ray diffraction and reflectivity measurements with synchrotron radiation. At the initial stage of superlattice growth, highly strained and smooth sublayers were found, indicating the repetition of two-dimensional nuclei and growth of the sublayer on the flat terrace of a SrTiO3 substrate; on increasing the bilayer number to N = 10, the growth front becomes rougher to relieve the lattice strain. Further increasing the bilayer number to N = 11, an abrupt increase of interfacial roughness is also followed. It corresponds to the effective critical thickness for pseudomorphic growth of a superlattice, beyond which surface and interface undulation result from the generation of interfacial misfit dislocations. (c) 2004 Elsevier B.V. All rights reserved.