Optical emission spectra of a CH4/H2/Ar gas mixture plasma were observed during carbon nanotube (CNT) growth in RF plasma-enhanced chemical vapor deposition. CNTs with diameters of ∼10-30 nm and length of ∼6 μm were grown on double- and triple-layered films of catalyst/support materials (FexOy/TiO2 and Al2O3/FexOy/Al2O3) at the total gas pressures of 1-10 Torr with gas flow rates of CH4 = 27 sccm, H2 = 3 sccm, and Ar = 1 sccm. The number density of CNTs increased with the gas pressure, and Al2O3/FexOy/Al2O3 (each thickness of 1 nm) film yielded the thinnest CNTs with a high number density among the present catalysts. The spatial distributions of H atom relative density in the plasma were obtained by actinometry. The H relative density decreased with the pressure, and this suggests the suppression of CH3 radical generation in the plasma.