High-quality a-SiC:H films have been prepared by using a hydrogen-radical CVD method. Si2H6 and C2H2 were used as source gases, and C2H2 was introduced by decomposing with microwave plasma. Consequently, carbon content and optical band gap could be controlled well by C2H2 the flow rate. It was confirmed that very stable a-SiC:H films can be produced by this technique.