The effect of electron-beam irradiation on the microstructural characterization of beta-FeSi2 thin film has been studied at room temperature by in situ transmission electron microscopy. Analysis of the captured video images reveals that significant changes occur in the microstructure of the heteroepitaxial phase during the irradiation. For heteroepitaxial layers with ultrathin thickness below 20 monolayers, the Moire fringe direction was observed to deviate from its original orientation and the islands often shrink and disappear under the electron-beam irradiation. These observations have significant implications for the microstructural characterization of ultrathin epitaxial films using transmission electron microscopy. (c) 2006 Elsevier B.V. All rights reserved.