Abstract: The α-Si₃N₄ ceramic green body was prepared by gel injection molding process, and was buried in the powder bed composed of BN, BN+Y₂O₃+Al₂O₃ (material ratio of 1∶1∶1), BN+Y₂O₃+Al₂O₃+Si₃N₄ (material ratio of 1∶1∶1), respectively, to prepare porous Si₃N₄ ceramics by pressure sintering process. The effects of buried powder composition on phase composition, microstructure, bending strength and dielectric properties of ceramics were studied. The results show that the porous Si₃N₄ ceramics obtained by sintering with BN buried powder were composed of α-Si₃N₄ particles with the highest porosity and the lowest flexural strength and dielectric constant. The ceramics obtained by sintering with BN+Y₂O₃+Al₂O₃ buried powder were mainly composed of granular α-Si₃N₄ and a small amount of elongated β-Si₃N₄ dispersed in the α-Si₃N₄; the porosity was relatively high, and the flexural strength and dielectric constant were relatively low. The ceramics obtained by sintering with BN+Y₂O₃+Al₂O₃+Si₃N₄ buried powder were composed of elongated β-Si₃N₄, and pores were formed by interlocking of β-Si₃N₄ grains, resulting in the lowest porosity and the highest flexural strength and dielectric constant. The dielectric loss of ceramics obtained by sintering with different composition buried powder were at the order of 10⁻³, indicating excellent microwave transmission properties. The porous Si₃N₄ ceramics sintered with BN+Y₂O₃+Al₂O₃+Si₃N₄ buried powder exhibited the best performance, with the porosity of 53.7%, flexural strength of 128.2 MPa, dielectric constant of 2.96, and dielectric loss of 1.62×10⁻³.