Abstract: The distribution of radicals in CHF₃/C₆H₆ electron cyclotron resonance （ECR） plasma was investigated by an actinometric optical emission spectroscopy （AOES）. The results show that the relative concentration of CF,CF₂,C₂ radicals increase with the increase of CHF₃ flow, while the deposition rate of a-C:F film presents a decreasing trend. The results also suggest that the relative content of C-F bonding in a-C:F film is closely related to the relative concentration of F atom in the plasma. The relationship between the precursor radicals and the deposition process of a-C:F film is discussed. It is pointed out that the gas flow rate of CHF₃/（CHF₃+C₆H₆） is an important factor in the a-C:F thin film fabrication.