By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/Si3N4) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/Si3N4 ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/Si3N4 ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure.