Strength and impact toughness of tempered Fe-Ni-C martensites with different substructures were studied. One (Fe-4.4%Ni-0.23%C alloy) was lath type martensite with dislocation substructure and the other (Fe-25.3%Ni-0.23%C alloy) was plate type martensite with internal twins at midrib. The impact toughness of Fe-Ni-C lath martensite showed a gradual and continued increase with tempering above about 100℃, while that of twinned martensite, after revealing a rapid increase at an early stage of tempering (up to about 160℃), showed no further improvement with increasing tempering temperature. The poor impact toughness of twinned Fe-Nice tempered martensite is considered to be due to the carbides preferentially precipitated at twin boundaries.