Effects of austenitization temperatures and holding time on magnetization, J (T), of Fe-17.5 mass%Cr-2.0 mass%Ni-0.8 mass%Al-0.5 mass%C based alloy were investigated in order to clarify the heat treatment condition for austenitizing and quantify the relation between magnetic properties and microstructures. The alloy exhibited high J value of 1.46 T after annealing at 1053 K. On the other hand, the J values of the alloy after austenitizing at temperatures between 1273 and 1473 K lowered with increasing austenitization temperature, describing s-shape curves. The J value of the alloy after austenitizing at temperatures above 1423 K for 240 s was nearly equal to zero magnetization. In addition, they also lowered with the increase of the holding time between 1 s and 240 s. The decrease in J values by austenitizing was closely related to the dissolution of the M23C6 particles into austenite (gamma) matrix phase, which proceeds at higher temperature for longer time. The increase in chromium and carbon concentration in gamma matrix phase due to the dissolution Of M23C6 particles resulted in the decrease in martensite start (M-s) and finish (M-f) temperatures, and the increase in the paramagnetic retained gamma fraction (X-gamma R). The following equation was introduced on the relation between magnetization and retained gamma fraction. J(gamma)/J(alpha) = 0.49X(gamma R)(2) - 1.50X(gamma R) + 0.99 J(gamma) means the J values of the alloy after austenitizing at temperatures between 1273 and 1473 K, while J(alpha) means that of the alloy after annealing at 1053 K (=1.46T). (c) 2006 Springer Science + Business Media, Inc.