A duplex stainless steel was aged at 723K for time periods up to 5000h to study the mechanism of the phase decomposition of ferrite. A time evolution of the compositional fluctuation was evaluated on the basis of the probabilities of the hyperfine magnetic field distribution computed from the Mössbauer spectra. Precipitation of the Cr-rich α' phase was investigated by the probability of the paramagnetic component and TEM observation.