Low pressure CH4, CH4 /H-2, and H-2 plasma beams were generated by a 13.56 MHz inductively coupled radio frequency (rf) plasma with a magnetic field (similar to300 G) to utilize them as radical sources for the epitaxial growth of diamond. The threshold ionization technique using a quadrupole mass spectrometer has been employed to measure the absolute density of CH3 radicals (n) in CH4 and CH4 /H-2 plasma beams. The ions of CH4 and CH4 /H-2 plasma beams were composed of CHx, C2Hx, C3Hx, and H-x fragment ions, while H-x fragment ions occupied the H-2 plasma beam. When the rf power and the pressure were increased, the it and the relative intensities of H-3(+), C2Hx, and C3Hx fragment ions were increased. This implies that both the homogeneous secondary ion-molecular reactions and the heterogeneous plasma-surface interactions take place in the plasma generation region of the low pressure plasma beams with an increase in rf power and pressure. The increase of H-3(+) also induces an increase of atomic hydrogen, which is essential for diamond growth. It is therefore concluded that moderate rf power and higher pressure are favorable for diamond growth.