The problem of polarity effect in EDM is researched in this paper.In order to find out deterioration of anode and cathode,experiment with single pulse is taken in kerosene oil.The deterioration of anode is found larger than that of cathede even in the condition of large pulse width,which is contradictory to deterionition in EDM machining with continuous pulses.This phenomenon above may be explained through analyzing the movement of electron and positive ion in plasma canal.