Nanometer-sized Xe particles embedded in Al are observed with an off-Bragg condition high resolution trans-mission electron microscopy. It is found that a Xe particle with size of about 1 nm shows quite different structural properties from those with larger sizes. It changes structure from an f.c.c. to another one, changes orientation with the same f.c.c. structure, or changes shape and sizes. These changes are attributed to (1) a high mobility of atoms on Xe/Al interface due to irradiation of 1 MeV electron beam and irradiation enhanced interfacial diffusion; (2) a smaller barrier in energy for shape change of a smaller Al void where the Xe particle is constrained; (3) a larger fraction of atoms on Xe/Al interface for a smaller Xe particle. (C) 2004 Elsevier B.V. All rights reserved.