Hydrogenated amorphous silicon carbide films (a -SiC:H) were prepared from CH4, SiH4, and Ar mixtures by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition (ECR PCVD). The deposition of the thin films was proceeded with the following optimized conditions; microwave power: 900W, Ar flux : 90sccm, and total flux: 113.4 sccm. The substrate temperature was around 100∼120°C during deposition. For comparisons, the relative flux ratio of methane to silane was varied to produce thin films of different compositions to investigate the relationships between the associated compositions of films and their corresponding microstructures and optical properties. Moreover, both film's microstructures and their optical properties were analyzed to find out as to how they are interrelated. Furthermore, the surface morphology and amorphous microstructures were confirmed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), respectively. And, x-ray Photoelectron Spectroscopy (XPS) was employed to study the relative atomic ratio of C to Si along with the bonding conditions in the thin films. Finally, the Hydrogen concentration and the amounts of C-H and Si-H bonds were determined by Fourier transform infrared spectroscopy(FTIR), while the optical properties were measured by optical spectrophotometer.