Abstract: In this paper the directional solidified grey iron of various carbon equivalent were thermodynamically analyzed, and the results show:under the condition of similar carbon equivalent, as the Si/C ratio increases the minimum eutectic solidification temperature of Fe-C-Si alloy rises,and the quantities of austenite dendrite are increased.Further analysis shows:the mechanism of increased quantities of austenite dendrites is that silicon makes the eutectic coupled zone move to the right in the Fe-C phase diagram.