Electron microscopy was used to investigate microstructures of diamond single crystals prepared at high temperature and high pressure (HPHT) from the Fe–Ni–C system. Analysis through selected-area electron diffraction pattern suggests that the crystal structure of the HPHT-grown diamond is cubic; polycrystalline diamonds are contained in the diamond. Etch pits on the (111) surface of the diamond by scanning electron microscopy revealed the dislocation motion under the action of applied stress. An array of parallel dislocation lines taken with reflection of  was observed directly by transmission electron microscopy (TEM). Fringe distortions and concentric dislocation loops were examined by rotation moiré images, which arose from two overlapping (111) close-packed planes rotated with respect to each other at a 5° angle. The parallel dislocation lines, distorted fringes, and concentric dislocation loops might have derived from the micro-inclusions. The parallel layers with growth cellular interface by TEM provided direct evidence that the diamond grew from solution of carbon in the molten catalyst at HPHT and the growth interface diamond was not stable.