Hydrogenated amorphous carbon (a-C:H) films and unhydrogenated amorphous carbon (a-C) films are promising as artificial joint coatings due to their excellent wear resistance. In this paper, the cytocompatibility of a-C:H film and a-C film was evaluated to determine the more appropriate type for prosthesis modification. The a-C:H film was deposited by electron convolute resonance plasma enhanced chemical vapor deposition (ECR-PECVD) and a-C film was fabricated by magnetic filtered cathodic vacuum arc (MFCVA). The microstructure and physical characteristics of the films were investigated by Raman spectroscopy, contact angle measurement, atom force microscopy (AFM), electrokinetic analysis, electrical resistivity measurement and Hall effect measurement. Bovine serum albumin (BSA) and serum protein adsorption on a-C and a-C:H films was determined by micro BCA assay. RAW264.7 macrophages were cultured on the a-C:H and a-C films for 24 h to evaluate the cell death, cell activation and inflammatory cytokine release. Primary mouse osteoblasts were cultured on the a-C:H and a-C films for 2 h, 1 day, 3 days and 6 days with biological tests performed to evaluate the cell adhesion, cell viability and cell morphology. The results show that a-C film is N type semiconductor with unpaired electrons. Protein adsorption assay shows that the a-C film can covalently bind more serum proteins than a-C:H film. The unpaired electrons of a-C film contribute to its better ability to covalently bind bioactive proteins than a-C:H film, and the superior adsorption and bioactivity of the adhesion proteins on a-C film further induce the better biological performance of a-C film. In a word, a-C film induces lower inflammatory reaction and higher ostoblasts viability than a-C:H film.