Abstract: It was observed experimentally that boron segregated to grain boundaries in Fe-40%Ni alloy during cooling process by boron autoradiograph technique and transmission electron microscope. The non-equilibrium segregation of boron on grain boundaries during cooling process from 1150 to 640 ℃ at 2 ℃/s shows three stages with different boron diffusion behaviors. The rich factors, widths of rich layer, depletion factors and widths of boron depletion zone were measured. The phenomena are discussed.