The magnetic coercivity of martensitic 12Cr steel was measured in order to evaluate its degree of isothermal aging. As the aging time increased, the lath width increased and the dislocations were recovered. Aging resulted in the coarsening of the as-tempered carbides (M23C6 and MX) followed by additional precipitation of Fe2W. The magnetic coercivity rapidly decreased during the initial aging period of about 1,000 hours and then decreased slightly thereafter. The decrease in the coercivity with increasing aging time was related to the decrease in the number of pinning sites, those associated with the dislocations, fine precipitates and coarsening of the martensite lath/subgrain.