It was shown by transmission electron microscopy (TEM) that there contain nanoscale crystalline inclusions about several nanometers in dimension in diamond single crystals synthesized in the presence of Fe70Ni30 alloy catalyst under high pressure and high temperature (HPHT). The inclusions distribute homogeneously within the diamond matrix, It was indicated by selected area electron diffraction (SAD) patterns combined with EDS analyses that the inclusions consist of three main types: hexagonal graphite with a lattice constant of a = 2.463 A, c = 6,714 A, f.c.c. (FeNi)(23)C-6 with a lattice constant of a = 10.89 A. and f.c.c. gamma-(FeNi) with a lattice constant of a = 7.146 A. (C) 2002 Published by Elsevier Science B.V.