The effect of the prior martensite on the thermal stability of the nano-bainitic steel was investigated. The nano-bainitic steel composed of prior martensite, nano-sized bainitic ferrite and retained austenite was obtained by quenching and followed by bainite transformation at low temperature. The microstructure and hardness variation were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and hardness tester etc. for the nano-bainite steels after tempering at different temperatures. Results show that after tempering at 473~773 K the hardness of the nano-bainite steel containing prior martensite is higher than that of the untampered ones. However, after tempering at temperatures above 823 K, its hardness decreased rapidly and which down to 266.2HV at 923 K. The carbon was discharged from prior martensite to the retained austenite when the steel was tempered at 473~573 K. The carbon content of the later increased to a peak value, i.e. 1.52%, which improved the thermal stability of retained austenite, and further delayed the decomposition of the later, thus improved the thermal stability of the nano-bainitic steel at high temperature. The retained austenite decomposed into carbides, and the bainitic ferrite coarsened and recovered, formed new ferrite grain when the tempering temperature exceeded 723 K.