The deposition of hydrogenated amorphous carbon (a-C:H)-based films at atmospheric pressure is a cost-effective process without using a high-vacuum technique. In this study, we synthesized a-C:H-based films by atmospheric pressure-plasma enhanced CVD method from C2H2/He and C2H2/N2 mixture gas and investigated the effect of dilution gas on the hardness of the films. The film deposited from C 2H2/N2 contained nitrogen elements with forming C-N bonds and the film was softer than a-C:H film deposited from C 2H2/He. The hardness of the film deposited from C 2H2/He was 1.1 GPa and twice higher than that from C 2H2/N2.