Synthetic diamond grains, produced under the static high pressure and high temperature (HPHT) conditions, were prepared into nanofilm for TEM observation by RTO method. It is found originally that diamond grain is actually consisting of many tiny rod-like nanodiamonds through an orientated piling or aggregation, and the amorphous carbon was filled in the area between these nanodiamonds. It meant that a synthetic diamond grain was constructed by bundles of nanodiamonds and amorphous carbon, and this originally finding inspired us to propose and draw a completely novel microstructure model for synthetic diamond grain, which could help us to better understand the anisotropy and other macroscopic properties of diamond grains. Based on this finding, it was further inferred that the traditional concept of monocrystal of diamond called in the superhard industry may not be rigorous or correct because it was probably not consisting of pure crystalline diamond.