Solid state reaction interdiffusion of Fe-Ti multilayer modulated films was investigated by means of dynamic in situ X-ray diffraction. With this method, the measurement of the X-ray modulation peak intensity was relatively accurate and the shift in X-ray modulation peak position was also observed. The interdiffusion coefficients of Fe-Ti modulated films were calculated at different temperatures. Plots of the interdiffusion coefficients versus time at different temperatures were given. According to the variation rate of D vs. τ, the activation enrrgy Qy relative to viscosity relaxation was calculated to be 8.3±0.2 kcal/mol.