Mechanical properties of SUS 321 (18 Cr-9 Ni-Ti steel) and 347 (19 Cr-9 Ni-Nb steel) aged at 700°C×75hr were studied at the temperatures below the room temperature, especially at liquid helium temperature. Cr rich carbides, M₂₃C₆, precipitated at grain boundaries in the both steels during aging whether the both were stabilized at 900°C beforehand or not. The distributions of grain boundary precipitates, however, were different. The aged SUS 321 showed both connected precipitates along grain boundaries and fine TiC scattered uniformly. On the other hand the aged SUS 347 presented isolated ones along boundaries as well fine Nb (CN) distributed uniformly. The degradation of tensile properties at 4K was small but that of fracture toughness was very large. The degree of degradation of SUS 347 was much smaller than that of SUS 321. Moreover 4K J_IC specimens of SUS 347 fractured transgranularly while those of SUS 321 did intergranularly. The results make it clear that the degradation of 4K fracture toughness due to aging could be suppressed by controling morphology of precipitates and that niobium addition is effective to make the morphology advantageous.