We will discuss the stress release phenomena, structural relaxation and interdiffusion processes during annealing. The [Co(4nm)/Ta(4nm)]38 multilayers were prepared by dc magnetron sputtering on Si substrate. The multilayers were annealed at various temperatures (523 - 673K) in vacuum (under 10-5 torr) furnace. The effective interdiffusion coefficients were determined from the slope of the best straight line fit of the first peak intensity versus annealing time [d ln(I(t)/I(0)) /dt] by X-ray diffraction (XRD) low angle measurements. The drastic decrease of the relative intensity in the initial stage shown due to the structural relaxation was excluded in the calculation of effective interdiffusion coefficients. The temperature dependence of interdiffusion in the range of 523 - 673K is described by D = 3.2×10-19 exp(-0.51±0.11 eV/kT) m2s-1.