Hydrogenated amorphous carbon nitride (a-C:N:H) films were synthesized with CH3CN dielectric barrier discharges (DBD) plasmas. The effects of varying the CH3CN pressure (p) and the frequency of the power supply (f) on the film growth and film properties were studied. The deposited films were characterized using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (AFM), and AFM-based nanoindentation. p and f were found to significantly influence the structures, compositions, deposition rates, surface roughness, and nanohardess of deposited a-C:N:H films. The experimental results indicate that dense a-C:N:H films with extremely low surface roughness (rms<1.0 nm) can be deposited with CH3CN DBD plasmas at f=1 kHz and p=∼100 Pa. The deposition systems were in situ characterized by means of optical emission spectroscopy. The emission intensities of major radicals, such as CN (B 2Σ→X 2Σ) and NH (A 3Π→X 3Σ) significantly increased with increasing f or decreasing p. N2 molecules were formed in the residual gas as a stable product, which leads to a decrease in the N/C ratio in deposited a-C:N:H films. The rotational and vibrational temperature of N2 C 3Π states in CH3CN and N2 DBD plasmas were examined and the N2 molecules produced in CH3CN DBD plasmas had the rotational temperature of ∼2000 K and vibrational temperature of ∼500 K. In the N2 DBD plasma, the rotational and vibrational temperature of the N2 molecules were 470±10 and 2850±50 K, respectively. The basic chemical reactions in the gas phase are presented and correlations between the film properties, the gas-phase plasma diagnostic data, and the film growth processes are discussed.