We have studied for the first time the structural change of high-purity metallofullerene (Gd@C(82)) upon heat treatment in an ultra-high vacuum system (10(-10) Torr) and examined the decomposition product through successive analysis with MS, IR, Raman, TEM, EDS and XPS. It was found that metallofullerene (Gd@C(82)) had fully collapsed at 580 °C which was lower than that for the complete destruction of C(60). The easier decomposition should be ascribed to the encapsulated metal in the carbon cage which could induce the deformation of the C-C bond. The analysis indicated that the broken metallofullerene (Gd@C(82)) became a kind of graphite-like material with a lot of defects. The Gd atoms leaked out from the carbon cage and aggregated together to form a regular arrangement.