the problem of polarity effect in EDM is researched in this paper. In order to find out the deterioration of the anode and the cathode, the experiment with single pulse is taken in kerosene oil. We find the deterioration of the anode is larger than that of the cathode even in the condition of large pulse width. But this result is contradictory with that gotten in electric discharge machining with continue pulses. Through analyzing the movement of the electron and positive ion in plasma canal, we explain the phenomenon above rationally.