It is an effective way to decrease dielectric constant of microwave transparent materials by improving porosity rate. Network -Si3N4 porous transparent materials were prepared by gas pressure sintering (GPS) with Y2O3 additions. Porosity rate of silicon nitride using pore former with different content starch, polytetrafluoroethylene (PTEE) powder respectively and their compounded component were discussed in this paper. It is concluded that porosity rate can be improved by all the three, starch is most favorable to improve porosity but great volume rebound after cold iso-static pressing were happened and easily lead to crack of green body. Method of pore-making by step on principle of pore formers discharging in different temperature range used to get high strength and porosity green body. Series of porous silicon nitride microwave transparent materials were prepared with porosity rate from 48% to 67%, bending strength from 162MPa to 59MPa and dielectric constant from 3.41 to 2.37.