A dielectric barrier discharge in Ar and CH4 gas mixtures used for a-C:H film deposition was investigated. The number densities of Ar(1s₂: resonant level) and Ar(1s_3,5: metastable levels) atoms were measured using a high sensitive detection method based on plasma modulation and laser absorption spectroscopy. The CH(X²Π) radical number density was measured by an actinometry method. The excitation transfer process from the Ar(1s₅) atom to the CH(A²Δ) radical was detected by a laser collisionally induced fluorescence method. The a-C:H film properties (the content of sp³ and sp² hybridization and the content of the CH₃ and CH₂ hydrocarbon groups) were correlated with the number densities of the Ar(1s₅) atom and the CH(X²Π) radical. In the a-C:H film, maximum value of the sp³/sp² was obtained, when the CH(X²Π) number density in DBD plasma was minimum.