Metal nanoparticles intercalated into and encapsulated inside single-walled (SWNTs) and double-walled (DWNTs) carbon nanotubes are synthesized using a plasma technique combined with the introduction of ionic liquids under low gas pressures. Owing to the synthesis in nano-spaces of the SWNTs and DWNTs as a template, high-density and monodispersed metal nanoparticles are realized, which could be applied to specific composite-nanodevices based on carbon nanotubes.