Ion energy distributions (IEDs) and the density of CH3 radicals (n) in a 13.56 MHz radio frequency (rf) low pressure inductively coupled CH4/H-2 Plasma used for nanocrystalline diamond deposition have been investigated with a quadrupole mass spectrometer. The energy distributions of positive ions were measured in a CH4/H-2 plasma with 50 mTorr of the gas pressure at 500 W of the plasma input power, and were compared with those of an Ar plasma. We have found that the IEDs of Ar+, CH4+, and C2H5+ have a nearly monoenergetic peak, and a hump due to a small degree of capacitive coupling. The plasma potentials obtained from the peaks are consistent with the previously reported values measured with a Langmuir probe. On the other hand, the IEDs of H+, H-2(+), and H-3(+) have a clear asymmetric double peak due to the modulation of rf driven glow discharge. The n monotonously increases with increasing pressure. The n indicates that CH3 radicals are main precursors for the growth of nanocrystalline diamond. The estimated sticking coefficient of the CH3 radical is comparable with the reported value. (C) 2003 American Vacuum Society.