Hexagonal, single-crystalline, diamond nanoplatelets synthesized by microwave plasma (MP)CVD on Au-Ge alloy and nanocrystalline diamond (nc-diamond) film substrates, respectively, are reported. On the nc-diamond matrix, hexagonal diamond nanoplatelets can grow to a thickness of as little as approximately 10 nm. The effects of various processing parameters, such as methane concentration, microwave power, and gas pressure, on the growth of diamond nanoplatelets are explored. High-resolution transmission electron microscopy (HRTEM) reveals that the diamond nanoplatelets contain multi-parallel twins, and the side faces of the platelets exhibit {100}/{111} ridge-and-trough structure. Anisotropic growth of diamond nanoplatelet is believed to result from the side face structure of the twinned platelets and intensive plasma reaction.