Thermal stability of the multilayer plays an important role in the applications. Co/Mo2C multilayer working at 778 eV was designed and fabricated by direct current magnetron sputtering, and the characterization was performed by using X-ray reflection and X-ray diffraction. The experimental results indicate great thermal stability with its period contraction. The as-deposited Co/Mo2C multilayer is in amorphous and has a good interface, while interfacial diffusion appears as the annealing temperature rising, and the Co-on-Mo2C interface is more stable than the Mo2C-on-Co interface due to Co3Mo crystalline alloy generated in Mo2C-on-Co interface.