The temperature dependence of austenite nucleation behavior was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel. The martensitic structure was partially reversed to austenite by heat treatment at different temperature in (austenite+ferrite) two-phase region. With increasing the reversion temperature, the shape of austenite grains tend to be changed from acicular to granular, and their nucleation site is changed from lath boundaries to prior austenite grain boundaries. The transition of nucleation site was discussed in terms of energetics by considering the increases in interfacial energy and elastic strain energy by formation of an austenite nucleus. The calculation results suggested that lath boundary is more preferential nucleation site rather than prior austenite grain boundary because the increment of elastic strain energy is reduced with lowing the reversion temperature.