Tungsten carbide thin films have been prepared by reactive rf sputtering from a tungsten target in various Ar-CH4 mixtures. The composition, structure, microstructure and chemical state of the films have been investigated by the complementary use of RBS, NRA, XRD, GIXRD, TEM and XPS analyses. These characteristics of the films were then correlated to their mechanical properties determined by hardness (H), Young's modulus (Er) and friction coefficient measurements. Under low CH4 pressures, the formation of a mixture of nanocrystalline WC1-x and W2C phases has been observed. A pure WC1-x, phase was observed in films having a composition close to W1C0.9. With increasing CH4 pressure, the amount of carbon in the films increases, leading to a progressive amorphization of tungsten carbide deposited layers. Nanocomposite films appeared to be formed, with WC1-x nanograins (< 3 nm) dispersed in an amorphous carbon matrix. The film deposited at 30% of CH4 exhibits a-C:H phase. The nature of the phases present in the films plays an important role on their mechanical properties, as shown by the wide domain of variation of the films' hardness (between 22 and 5.5 GPa) and the plastic deformation parameter H-3/E-r(2) (between 0.08 and 0.04).