We have grown thin Ni films with various thicknesses on polished MgO(100) single crystal substrates in an e-gun evaporation system. The morphology of the as-deposited films was characterized with atomic force microscopy. Pinholes with average diameter of 5-10 nm are found in the film with thickness from 1 to 15 nm, and pinholes array was observed in the film 10 nm thick. The origin of such structure formation is discussed in terms of the elastic strain energy. (C) 2003 Elsevier B.V. All rights reserved.