The effect of austenitizing temperature and holding time at different austenitizing conditions on the behaviors of transformed austenite phase from ferrite in ductile cast Iron has been investigated. The formation behaviors of bainite formed by destablized isothermal heat treatment at 300℃, 400℃ after heating in the eutectoid transformation range(α+γ) for 1 hour were also, studied. Even the volume fraction of transformed austenite was same MVH(Micro Vicker`s Hardness) of austenite formed at high austenitizing temperature has higher value than that of austenite at low temperature. The bainite phase began to form at interface of austenite and ferrite which carbon concentration is low relatively. This difference in the behaviors of transformed austenite seemed to be affected by the segregation of substitutional alloying elements and the change of carbon diffusion rate with various austenitizing conditions and austempering conditions.