Compositionally gradient CrN x coatings were fabricated using arc ion plating by gradually increasing N 2 flow rate during the deposition process. The effect of substrate bias, ranging from 0 to −250V, on film microstructure and mechanical properties were systematically investigated with XRD, SEM, HRTEM, nanoindentation, adhesion and wear tests. The results show that substrate bias has an important influence on film microstructure and mechanical properties of gradient CrN x coatings. The coatings mainly crystallized in the mixture of hexagonal Cr 2 N, bcc Cr and fcc rock-salt CrN phases. N 2 flow rate change during deposition results in phase changes in order of Cr, Cr+Cr 2 N, Cr 2 N, Cr 2 N+CrN, and CrN, respectively, along thickness direction. Phase fraction and preferred orientation in CrN x coatings vary with substrate bias, exerting an effective influence on film hardness. With the increasing of bias, film microstructure evolves from an apparent columnar structure to a highly dense one. The maximum hardness of 39.1GPa was obtained for the coatings deposited at a bias of −50V with a friction coefficient of 0.55. It was also found that adhesion property and wear resistance of gradient CrN x coatings were better than that of homogeneous CrN coatings.