The behaviors of amorphous W/C periodic multilayers after annealing have been studied by precise low-angle X-ray diffraction. Afte refraction correction, the period of multilayers can be calculated more accurately using th low angle peaks. The period increases with increasing annealing temperatures. The intensities of 2nd and 3rd Bragg diffraction also increase with the annealing temperatures. These phenomena can be interpreted by the increase of thickness of the carbon layers.