SF6 gas insulated switchgear plays an important role in electric power networks all over the world due to its merits as compared to traditional air insulated switchgear. According to a numerous studies, it appears very difficult that any pure gas can bring a solution to the issue of desirable insulation ability and low environmental impact, so the mixtures composed of a strongly electronegative gases with high dielectric strength such as SF6 and ordinary gas (N2, CO2, or Air) are used to reduce the gas price and liquefaction temperature. From this point of view, various types of gas mixtures are used inside gas insulated switchgear (GIS) and gas insulated lines (GIL) to give a higher dielectric strength with lower cost and lower environmental impact. In this paper, the Finite Elements Method (FEM) is used to evaluate the potential and electric field distributions on and around multi contaminating particles. Two cases are studied in this work, the first is that there are three contaminating particles rested at ground plate, and the second case is that there are two particles, one of them is rested on the ground plate and other is hovering inside the gap between two parallel plates. The effect of spacing between the particles on the electric field values is studied. The breakdown voltage calculations under uniform field in case of clean gap and non-uniform field in case of gap with multiparticles contamination are studied. The effect of gas pressure, SF6 gas concentration in mixture and the spacing between particles on the breakdown voltage calculations are also studied.