This paper describes about development of the alloy cast iron which can be used as the cutting edges of a trimming die of press die. Usually, the block of a tool steel or steel casting is inserted to the cutting edge of the trimming die of a press die. However, we unified the structure part and the cutting-edge part of a press die with alloy cast iron. Of course, in this state, since it can't bear as the cutting edge, the cutting edge is processed in flame-hardening. After the flame hardening, we developed the alloy cast iron so that hardness enough by the natural air cooling might be obtained. Thereby, the machining of the installation seat of the cutting edge decreased and the expenses on dies has been reduced.