The interdiffusion behavior of W coating with Mo alloy at 1600 ℃ was studied, the monocrystal samples remained monocrystal feature without after long time (maximum of 5000 h) annealed, and grain boundary of polycrystal W was perpendicular to the surface of Mo. As the result of line scan of SEM: the diffusion depth of W in Mo was larger than that of Mo in W. The interdiffusion coefficient D of monocrystal Mo-W system was calculated with EPMA datas by the method of Den Broeder, and as the result: D was 10-15~10-16 cm2/s at 1600 ℃, lnD was linear relationship inverse of concentration of W, and the slope was –1.82.