Tempering behavior of very low carbon (about 0.01%) Fe-Ni martensitic alloys was studied. For this purpose, two Fe-Ni alloys having different martensite substructures were prepared: one was Fe-18% Ni alloy of lath martensite and the other was Fe-31.7% Ni alloy of plate martensite plus small amount of retained austenite. Optical microstructures and transmission electron micrographs were observed to identify any carbide precipitation and substructural changes during tempering. Hardness and tension tests were performed and the mechanical property change with tempering was discussed.