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Remote plasma is an efficient method to synthesize carbon nanomaterials because it can avoid the damage due to ion bombardment and can supply active precursor species for their growth. We introduced dc plasma to the CH4 thermal chemical vapor deposition (CVD) of carbon nanotubes (CNTs) in order to decompose CH4 efficiently. The effect of the plasma was investigated through the gas contents observed by a quadrupole mass spectrometer and the quality of CNTs evaluated by Raman spectroscopy. High-quality single-walled CNTs grew on Al2O3/Fe/Al2O3 substrates, on which CNTs did not grow in conventional thermal CVD. The intensity ratio of the G peak to D peak in Raman spectra (G/D ratio) was 41.8. Since the CNT yield increased with the discharge voltage, the activation of precursors by the plasma seemed to promote the CNT growth significantly. At this time, C2Hx and C3Hx, produced in the plasma, also increased. Thus, they seemed to contribute to the CNT growth. The diameter of CNTs, as well as the CNT yield, increased with increasing discharge voltage. This indicated the presence of the optimum precursor supply rate depending on the size of catalyst particles to keep them active.