We have studied the structural properties of hydrogenated carbon films deposited by plasma enhanced chemical vapor deposition (PECVD). The substrate holder in reaction chamber could be biased and be heated. The Raman peak intensity at 1350 cm−1 was increased by reducing CH4 flow rate. The film structure changed from soft a-C:H to hard carbon with decreasing CH4 flow rate, resulted from increased self-bias. The 1520 cm−1 peak shifts to higher frequency by reducing the CH4flow rate, probably resulted from the increased internal stress.