Microwave discharge characteristics and variable plasma conductivity play an important role in design of microwave filters and the efficiency of radars and electronic warfare devices. A three dimension model was designed to study the effect of plasma formation on high power microwave (HPM) transmittance in cavity resonator based on COMSOL platform. The effect of gap length, electrode shape and size, coaxial port dimensions and gas pressure, on the electric field in the gap and insertion loss in cavity were investigated. The effect of the cross-section of coaxial port on the electric field created in the gap was investigated and the best values were calculated and determined. The electron density profile, electron temperature, electric conductivity, elastic and inelastic collision frequency were calculated. The performance of plasma limiter was optimized. Results were compared with the experimental data.