Wave-transparent materials used at high temperature environment generated by high supersonic and hypersonic speeds must possess excellent mechanical property. In this paper, porous Si3N4 ceramics with high strength were fabricated by low molding pressure (10 MPa) and pressureless sintering process, without any other pore forming agents. The sintering behavior and the effect of porosity on the mechanical strength and dielectric properties were investigated. The flexural strength of porous Si3N4 ceramics was up to 57-176 MPa with porosity of 45-60%, dielectric constant of 2.35-3.39, and dielectric loss of 1.6-3.5 x 10(-3) in the frequency range of 8-18 GHz, at room temperature. With the increase of porosity, the flexural strength, dielectric constant, and dielectric loss all decreased.