Silicon carbide (SiC) encapsulated with graphitized nanodiamond (GD) sheaths (SiC-GD) as a core-shell nanocrystal (NC) is synthesized with atomic H post-treatment of vertically aligned carbon nanotubes, which are unzipped and converted into graphene nanoribbons (GNRs) and preserve their vertically aligned integrity. The SiC-GD core-shell NCs anchor on the GNRs framework and form SiC-GD@GNRs composite. The nucleation and growth mechanisms for single crystal nanodiamond without diamond seed are discussed. The SiC-GD@GNRs composite as a metal-free electrocatalyst exhibits a high activity in the hydrogen evolution reaction, with a small onset overpotential of 8 mV, Tafel slope of 54 mV dec(-1), and exchange current density @200 mV of 77.4 mV, as well as long term stability in acid medium. The superior electrocatalytic performances of SiC-GD@GNRs are ascribed to a high dispersion of SiC-GD NCs on the GNRs support and a high stability of the GD and GNRs in acid solution.