Abstract: The high toughness ductile cast iron directly obtained at as-cast state had been studied. It was found that the content of Si and Mn were the main factors affecting the amount of ferrite and the value of δ% under the good state of graphite spheroidization. The value of δ% raised with the increasing in amound of Si and the decreasing in amount of Mn. When the amount of Si was in the range of (2.90-3.20) wt% and Mn in the range of (0.15-0.40) wt%, the value of 6% reached (19-21)%. This kind of as-cast high toughness ductile cast iron has been successfuly used in producing heavy section castings and thin section castings.