The volume fraction, morphology and size of reversed austenite in 5.5Ni steel tempered at different temperatures were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope, and the influence of tempering temperature on mechanical properties of 5.5Ni steel was investigated. The results show that there was no significant change in either the tensile strength or yield strength for the steel tempered in the range from 580℃ to 600℃. There was a slight increment in tensile strength but a great decrement in yield strength, besides, a maximum elongation was obtained for the steel tempered at 620℃. As the tempering temperature increased from 580℃ to 620℃, the volume fraction of reversed austenite in 5.5Ni steel increased gradually but impact energy decreased. Stable, homogeneous, dispersive and fine lamella-like reversed austenite is the main reason of the high impact energy of 148 J when the steel tempered at 580℃. Two types of reversed austenite including lamella ones and block ones were detected in this steel. The former had different length with a width of about 20 nm which could improve the low temperature toughness of the steel. The latter had a size of about 200 nm and tended to gathering together as clusters which were detrimental to the low temperature toughness of the steel.