a-C∶F∶H films are prepared by microwave ECR plasma-enhanced chemical vapor deposition method at variable CHF3/CH4 gas flow raitos. The results from the Fourier transform-infra-red abserption for these films have shown that a structural evolution of the a-C∶F∶H film occurs at variable flow ratios R =CHF3/(CHF3+CH4). The main structure of the films is diamond-like carbon (DLC) characteristics for R less than 64%. The film presents a structure of PTFE-like as R is larger than 64%, where the dominant structural monomer is —CF2. Meanwhile, this structural evolution has also an influence on optical gap of the films. Optical gap decreases with the increase of flow ratios in the region of DLC-like characteristics, while increasing in the PTFE-like region. The transmittance of the a-C∶F∶H film is close to 100% for R larger than 92%.