Abstract: Different heat treatment processes were carried out on Fe-44%C ultra-high carbon steel as follows: direct water quenching, isothermal quenching at 150 ℃, 175 ℃, 200 ℃, 225 ℃ and 250 ℃ for different times and annealing treatment. The microstructure after isothermal quenching and the formation model of bainitic sheaf during isothermal holding were studied mainly. The results show that independent dispersed bainites formed when the tested steel was holded at less than 200 ℃, and bainitic sheaf formed at over 200 ℃ and the evolution of a bainitic sheaf was that the new bainitic rods nucleated and grew at the both sides of prior formed bainitic rod. The bainite rod had higher carbon solubility than the pearlite ferrite, and the carbon solubility in bainite rod increased with the decrease of isothermal temperature.