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The WCN films with different C contents were deposited under different substrate bias by a sputtering system. The composition, structure and mechanical properties were characterised by X-ray photoelectron spectroscopy, X-ray diffraction, high resolution transmission electron microscopy, scanning electron microscopy, atomic force microscopy and nanoindentation. Results showed that the WCN films consisted of WCN, W 2 N, amorphous C and CNx phases. With the incorporation of C content, the growth pattern changed from continuous columnar to discontinuous columnar due to grain refinement. By applying substrate bias to the films, the growth pattern changed from discontinuous columnar to fine grained structure because the bias could enhance the mobility of impinging ions or atoms leading to film densification. As increasing the C content, the hardness first increased and then decreased due to the effect of solid solution strengthening and grain refinement. However, the substrate bias had little influence on the hardness because of the co-action of the dense structure and residual stress induced by bias.