Abstract: Cr-doped graphite-like amorphous carbon films were deposited by unbalanced magnetron sputter ion plating technique at different substrate bias voltages. Influences of substrate bias voltage on microstructure, hardness, internal stress, bonding strength and carbon element′s valence bond structure of the films were investigated, and the influence mechanism of substrate bias voltage on hardness was analyzed emphatically. The results show that the microstructure of the films gradually became compact initially with the increase of substrate bias voltage, and then damaged flocculation structure comes into being as the bias voltage was too high. The hardness, internal compressive stress and bonding strength of the films increased first and then decreased with the increase of substrate bias voltage. The films deposited at -90 V possessed the highest hardness (3 295 HV) and internal compressive stress (1.133 GPa) as well as the finest microstructure and the most sp3 carbon content. The films deposited at -65 V showed the highest bonding strength (Lc=53.8 N). The changes of hardness of the Cr-doped graphite-like carbon films were closely related with the evolutions of carbon element′s valence bond structure, internal stress and microstructure of the films.