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.