Atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) has attracted much attention for its cost-effectiveness owing to eliminate the use of vacuum devices. We synthesized hydrogenated amorphous carbon (a-C:H) films under atmospheric pressure from C2H2 gas diluted with N2 with varying pulse frequency of plasma source. We investigated the effect of surface texture and chemical bonding structure of the films on hardness. The hardness, surface roughness and chemical content ratio were analyzed by tribo scope nano-mechanical indentation tester, atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. As the pulse frequency decreased from 10 to 2kHz, the hardness increased from 0.35 to 0.92GPa and the surface roughness decreased from 49.8 to 14.3nm. From the result of XPS analysis, the N/C molar ratio increased from 0.022 to 0.094 with increasing the pulse frequency. © 2012 Elsevier B.V.