The interdiffusion of Mo-Si and Ti-Si multilayers is investigated by an in situ X-ray diffraction technique. It is found that the behavior of the interdiffusion in both multilayers is quite different. Both positive and negative interdiffusion take place in Ti-Si multilayers during annealing, while only positive interdiffusion occurs in Mo-Si multilayers. Accompanying interdiffusion, the process of solid state amorphization reaction plays a dominant role in Ti-Si multilayers, on the contrary, the crystallization process was the main process in Mo-Si multilayers. In addition, a transient amorphization process at early stage of annealing is observed in Mo-Si multilayers for the first time.