A composite material, made of carbon nanotubes (CNTs) partially embedded in a nanocrystalline diamond film was produced. The diamond film was first decorated with palladium or nickel nanoparticles. An array of nanopores was drilled in the film in a hot filament CVD (HFCVD) reactor thanks to the anisotropic etching that takes place under the nanoparticles. During this etching process, the metallic particles penetrate the diamond film to a controlled depth, thus remaining at the bottom of the nanopores. The buried nanoparticles remain catalytically active and are used to grow a multiwall carbon nanotube forest using HFCVD in the same reactor without breaking the vacuum. The quality of the CNTs was assessed by scanning electron microscopy and Raman spectroscopy. The interface between the carbon nanotubes and the diamond was characterized by ultrasonication, lateral force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. As a result of these characterizations, we demonstrate that the buried carbon nanotubes exhibit higher mechanical stability and improved electrical behavior compared to CNTs directly grown on the diamond surface.