TEM observation was originally presented about the interior microstructure of a diamond particle, and three different kinds of carbon allotropes were found existing in diamond. The synthetic diamond particle was constituted of many tiny columnar monocrystals with the approximate diameter of 10 nm and length of more than several hundred nanometer. These nanocrystals were assembled into clusters along the < 111 > lattice plane, while between these nanocrystals there were the amorphous carbons filled with, and this study originally revealed the microstructure of the synthetic diamond particle. Internal structure with crystal defects was also demonstrated clearly. These findings show the interior microstructure more explicitly, which may give useful inspiration to the technical progress of diamond synthesis. (C) 2016 Elsevier B.V. All rights reserved.