[en] Fe-Ni martensitic alloys being available for base alloy of strength alloy steels were controlled to have three kinds of microstructure such as lath martensite, plate martensite and mixture of them, by adjusting Ms temperature due to addition of nickel. And then effect of the microstructure on damping capacity of the Fe-Ni binary system was studied. The damping capacities of the Fe-Ni martensitic alloys were increased with maximum surface strains, And the damping capacity of the lath martensite with substructure of dislocation was higher than that of the plate martensite having substructure of internal twin. From this result, it was concluded that the dislocation substructure has greater influence on the damping capacity than the substructure of internal twin in case of non-thermoelastic martensitic alloy. On the other hand, the damping capacity of austenite was higher than that of the martensite in Fe-31% Ni and Fe-33% Ni alloys. This was because the magneto-mechanical damping capacity of the former was higher than that of the latter. The damping capacity of mixed phases of the austenite and the martensite was decreased linearly with increasing martensite volume fraction. In other words, the damping capacities in Fe-Ni alloys were in good agreement with rule of mixture.(Author)