The a-C:F:H films were prepared with the mixture of CH 4 and CHF 3 gases by the microwave ECR chemical vapor deposition method. The deposition rate of the film first increases and then decreases with variable flow ratios R {[CHF 3 ]/([CHF 3 ]+[CH 4 ])}. The results from Fourier transform infrared transmission spectroscopy for these films have shown that a structural evolution of the a-C:F:H film occurs with the variable gas flow ratios R. The main structure of the film presents a DLC-like characteristic as R<64%, while the film takes on a PTFE-like structure as R>64%, where the films consist mainly of -CF 2 - chains. It seems that this structural evolution also exerts an influence on the optical band gap. The optical band gap decreases with the increase of flow ratios R in the DLC-like region, while it increases in the PTFE-like region. Hydrogen and fluorine concentrations of these films from FTIR are also discussed.