The particle contamination inside the Gas-Insulated Systems (GIS), plays a major role to occurrence the Partial Discharge (PD), which causes the insulation failure in the GIS. This paper presents the experimental and theoretical study of breakdown voltage initiated by a metallic particle inside a gap insulated by compressed SF6 gas. Study the effect of particle contamination on the electric field distribution in the gap. Study the effect of gas pressure, gap spacing, particle shape and the particle size on the breakdown voltage that initiated by the metallic particle. Study the Particle Contamination Factor (PCF) to describe the sensitivity of the dielectric strength at the contaminating gaps. The electric field calculated by the Finite Element Method Magnetics (FEMM) software, which is based on using the Finite Elements Method (FEM). The breakdown voltage initiated by the particle contamination is calculated by the streamer formation criterion. The experimental study carried out in the laboratory with using the pressure vessel and negative high voltage DC source. The measured breakdown voltage for different particle size agreed well with the calculation of breakdown voltage.