Abstract—Non uniform gap like rod-plane gap and its electric field distribution has a great importance in high voltages. The maximum electric field of rod-plane gap is strongly affected by geometry (shape and dimentions) of two electrodes and the distance between them. The Finite Element Method (FEM) has been used throughout this work, for its favorable accuracy, to calculate the electric field distribution for rod-plane gaps.This paper studied the effect of the rod diameter, the plane diameter and the gap space on the maximum electric field of rod-plane gap. Influence of wire and cone particle contamination that settled on the grounded electrode on the electric field distribution of rod-plane gap is carefully researched in this paper as well. Effects of wire particle length and radius on the maximum electric field are also studied. Finally, effect of cone particle length on the maximum electric field is implemented.