The synthesis of single-crystalline diamond nanorods (with diameters of 4-8 nm and with lengths up to 200 nm) via the hydrogen plasma post-treatment of multiwalled carbon nanotubes is described. The diamond nanorods (see Figure) are identified as having a core-sheath structure with the inner core being diamond crystal and the outer shell being composed of amorphous carbon. A growth mechanism for diamond nanorods is proposed.