The structure and lattice parameters of Xe particles about 1 nm to about 6 nm in size embedded in Al were investigated with off-Bragg condition high-resolution transmission electron microscopy. An Xe particle about 1 nm in size had different structural properties from those 2-6 nm in sizes. Some 1-nm Xe particles had an face-centred cubic (f.c.c.) structure with the same orientation as the Al matrix, whereas others of the same size had a non-f.c.c. structure. The lattice parameters of a 1-nm f.c.c. Xe particle were about 20% smaller than the average value obtained from electron diffraction, i.e. the particle was compressed by about 80%. The lattice parameters of Xe crystals about 2 nm to about 6 nm in size were almost the same as those obtained from diffraction results. One of the reasons for the extra compression seen with a 1-nm Xe particle is the increase in pressure inside an Xe particle with decreasing particle size.