Amorphous carbon on tungsten (a-C/W) bi-layers have been deposited, on crystalline silicon substrates, by Radio-Frequency (RF) sputtering. Carbon films were deposited, from a high purity graphite target, with a RF power of 250 Watts; while those of tungsten were obtained, from a pure tungsten target, at two different RF powers of 100 and 200 Watts. Annealing effects, at 1000 K in vacuum, on the structure and the interface state of the samples, were studied by Grazing Incidence X-rays Reflectometry (GIXR) and Diffraction (GIXD) techniques. The GIXR measurements show that the raw a-C/W interfaces are abrupt while the annealed ones are diffuse with formation of a W2C interfacial layer. The GIXD measurements reveal that W films deposited at 100 Watts are amorphous whereas those deposited at 200 Watts are polycrystalline in the -W phase. After annealing at 1000 K, the formation of W2C at the a-C/W interfaces is confirmed by GIXD measurements. The W2C content is more important in the case of the W layer deposited at 200 Watts. Moreover, the part of the W layer, which has not reacted with the a-C layer, is crystallized in the α-W phase after the annealing.