The crystalization process of Si-W alloy film and the change of interfaces of Si-W poly-Si/SiO2/Si(100) after annealing have been studied by cross-sectioned TEM method. After annealing at above 440℃, nucleation of crystalline WSi is uniform in the alloy film, i.e., there is no preferential nucleation at the surface and interface. When the annealing semperature is lower than 800℃, the crystallization reaction take place in the alloy film, surface and interface of alloy film remain almost unchanged. After annealing at 800-1000℃, small thermal grooves appear on the surface of WSi. Very small fluctuation appears at the poly-Si/SiO2 interface as observed by using high resolution electron microscopy, but WSi/Si interface becomes quite rough. We propose that interdiffusion of Si and W atoms. in the interface driven by the interface tension is the cause of interface roughness. The interdiffusion of Si and O atoms is difficult due to the strong Si-O bond in SiO2, so the Si/SiO2 interface keeps nearly flat.