Amorphous carbon nitride thin films are deposited by cathodic are ion plating at the de current of 60 A and the nitrogen pressure of 1 Pa with various negative bias voltages. Structure, chemical composition and chemical bonding states are analyzed by using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy. The deposition rate and the N:C ratio change with bias voltage. The N:C ratio decreases with negative bias voltages. Application of negative bias on the substrate is effective to increase the film hardness. The maximum hardness is obtained at about -300 V. By this negative bias the three dimensional C-N bonding develops and the film becomes hard.