This paper investigates pressure influence on polycrystalline diamond formation using sawtooth-waveform modulated induction thermal plasma. Modulated induction thermal plasma was used to promote nucleation of diamond particles in the first stage. The operating pressure was set to 32, 60, and 90 Torr. The deposited diamond films were analyzed by field emission scanning electron microscope and Raman spectroscopic observation. Experimental results indicated that a lower pressure condition provided a higher deposition rate and a qualitatively better diamond film. Spectroscopic and high-speed video camera observation showed that a lower pressure condition expanded the thermal plasma flow axially onto the substrate. Numerical simulation was also made for Ar/CH 4/H 2 induction thermal plasma to study the influence of pressure on the thermal plasma flow. Calculation results showed that lower pressure involves a higher particle flux of neutral hydrocarbon species on to the substrate surface because of higher convective transport of these species before their ionization. These results imply that neutral hydrocarbon would play a more important role for diamond deposition than hydrocarbon ions in the present thermal plasma diamond film deposition.