Density changing with substrate negative bias of tetrahedral amorphous carbon (ta-C) films deposited by filter cathode vacuum arc (FCVA) system was studied by X-ray reflectivity technique. According to the dependence of film density on substrate bias, it is found that the film density reaches a maximal value of 3.26 g/cm3 at -80 V bias. As the substrate bias moves up or down, the density tends to decrease gradually. When the substrate bias was set to a high value as -2000 V, the density of ta-C film falls to be 2.63 g/cm3, which does not change much comparing with the maximum. Furthermore, sp3 content of ta-C films was obtained from their corresponding density and the maximal value of sp3 content can be extrapolated to more than 80%.