A nanocrystalline diamond (NCD) grown on carbon fibers substrate (CF), has been developed for electric double-layer capacitor. Carbon fibers were treated at 1300 and 2300 K by using the temperature steps of 60 K/h in a nitrogen atmosphere. NCD films were grown from Ar/H-2/CH4 mixtures on a hot-filament chemical vapor (HFCVD) deposition reactor. Scanning electron microscopy (SEM) images of NCD showed faceted diamond grains for both substrates. Raman spectra are characteristic of NCD films and confirm the existence of sp(2)-bonded carbon in grain boundaries due to significant reduction of grain size. NCD/CF samples showed the characteristic behavior of an ideal current-potential capacitor with rectangular current-potential responses curves in 0.5 M sulfuric acid. The NCD/CF composite treated at 1300 K has the largest cathodic current and retains the rectangular-shaped CV up to a high scan rate of 100 mv/s. (c) 2007 Elsevier B.V. All rights reserved.