The local environment issued from the phase decomposition in Fe-25Cr-12Co-1Si alloy during step ageing has been studied by 57Fe Mössbauer spectrometry. The phase decomposition is influenced by both ageing time and temperature. It processes rapidly when ageing temperature is high. The formation of different kinds of (Fe, Co)-rich, Fe-rich and Cr-rich domains with independent hyperfine parameters during phase decomposition was characterized by hyperfine field distribution. The magnetic hardening of the alloy was discussed basing on single domain particle model.