The correlations between microstructures and magnetic properties were studied in four Fe-17.5 mass%Cr-2.0 mass%Ni-Xmass%C (X = 0.3-0.6) alloys. Each alloy consisted of ferromagnetic alpha and M23C6 carbide phases by step- annealing at 1053 K and 848 K. The number of the M23C6 carbide particles increased with the carbon content, and this microstructural variation caused a deterioration of the soft magnetic properties. On the other hand, the alloy with 0.3 mass% carbon content consisted of ferromagnetic alpha' and paramagnetic retained gamma structures after solution treatment at 1473 K. The amount of the stable gamma structure increased with the carbon content, while the amount of the alpha' structure decreased. This microstructural variation caused a decrease in the relative permeability, mu(r), and the stabilization of the paramagnetic property to low temperatures below room temperature. The temperature stability of the mu(r) values was closely related to the martensite start temperature, M-s. From an equation for the estimation of Ms from the chemical compositions of the gamma phase, the M-s's of the alloys with 0.3, 0.4, 0.5 and 0.6 mass%C were estimated to be 326, 289, 241 and 216 K respectively. These values were consistent with the Ms's expected from the microstructures and temperature dependences of the mu(r) values. (C) 2003 Kluwer Academic Publishers.