More researchers hme paid mltch attention to fabrication of field emission triode with volcano-typed gate on the silicon substrate in recent years because of ease of fabrication and low cost. In this paper, five different structures of this triode are presented The electric field of the top of field emitter(Etip) is calculated in these structures by using EMAS software, and the different potential distribution and electric field distribution are got from these calculations. The results show that the diameter of gate hole gives much effect of the Etip fof" this triode, Because the volcano-typed gate holes fabricated by wet or dry etching have the very sharp rim, when the space between anode and gate is small, the high electric fieM more than 1