A pulsed laser-atom probe has been used to investigate microstructural change and phase separations that occur during 475°C embrittlement of ferritic stainless steel. In this study the Fe-28% Cr-5%Ni alloy was aged at 450°C for as long as 5000h. In addition to the atom probe analysis, hardness, the fatigue crack propagation rate and Charpy V-notch impact-energy absorption were measured. We have found that the amplitude of the initial Cr concentration fluctuation increases with ageing up to 1035h and the alloy aged at 450°C undergoes a spinodal decomposition. The size of Cr-rich precipitates increases with ageing time up to 5000h, while the wave length is saturated to 18∼32A^^○ at the ageing time of 1035h. The Cr-rich phase formed is considered as the major factor for the embrittlement.