A synthesis method that provides high‐purity diamond films is proposed employing direct current arc discharge plasma jet chemical vapor deposition. In the method, an electric current was supplied to a plasma jet stream by applying a bias voltage between a cathode and the substrate on which diamond films were deposited. The Raman spectral analysis showed that the purity of the synthetic diamond was remarkably improved by the application of the bias voltage during deposition. The alternating current calorimetric method was employed to measure the thermal diffusion coefficient of the synthesized diamond films. The thermal diffusion coefficient greatly increased for films deposited with biasing. This improved thermal diffusion coefficient suggests higher purity diamond films. Emission spectral analysis revealed that the quantity of the dissociated hydrogen contributing to the plasma emission near the substrate is greater when biasing is used. Thus, the dissociated and excited hydrogen atoms are considered to play a key role in removing the carbon allotropes other than diamond. ©1995 American Institute of Physics.