Hard diamond-like carbon (DLC) films and tetrahedral amorphous carbon (Ta-C) films have been deposited on X-Y cut quartz substrates. The 1-mum-thick DLC films prepared by a plasma enhanced CVD system possess typical Young's modulus (and hardness) of approximately 150 (19) GPa, whereas the 0.12-mum-thick Ta-C films grown using filtered cathodic vacuum arc technique have a Young's modulus (and hardness) of 340 (38) GPa. Velocity tests of surface acoustic wave (SAW) were performed by introducing SAW into two delay lines on the quartz; one coated with the carbon films and the other free of the films. It has been confirmed that the DLC and Ta-C films speed up SAW by 2.3% and 3.4%. respectively. These findings are interpreted in terms of the film properties.