The magnetic coercivity of ferritic 12Cr steel was experimentally studied in order to characterize its microstructures and mechanical properties during isothermal aging. As the aging time increased, the M23C6 carbide coarsened and additional precipitation of Fe2W phase was induced. The width of martensite lath increased to about 0.4μm after 4000 hrs of aging. The coercivity decreased as the number of precipitate decreased and the width of martensite lath increased. The hardness was proportional to the magnetic coercivity. These empirical linear relations suggested that the change in the microstructures and strength of ferritic 12Cr steel during thermal aging could be evaluated by monitoring the magnetic coercivity.