Silicon nitride porous ceramics possessed excellent mechanical， dielectrical， thermal properties and chemical stability. It was very suitable for broadband wave transparent materials using in high temperature， large load and strong corrosion circumstance. Reaction bonded silicon nitride porous ceramics had advantages of excellent properties， convenient process and low cost. The characteristics of Si powders remarkably controlled the phase component， microstructure， mechanical and dielectric properties. In this study， Si powders with different particle size and purity were used to prepare gelcasting and reaction bonded silicon nitride porous ceramics. The results show that dual granularity produces close packing effect in the green body， which is inherited and further evolves into a reinforcing and toughening mechanisms of two-tier microscopic tissue in the sintered body. Dual granularity of 5 μm and 45 μm obtains the largest bending strength of 109.94 MPa and work of fracture of 990.74 J/m². The values are 111.42% and 25.97% higher than that of single granularity of 5 μm as well as 46.55% and 20.46% higher than that of single granularity of 45 μm . The dielectric constant and dielectric loss are about 4.20 and 0.007 respectively. Gelcasting and reaction bonded silicon nitride porous ceramics can possess both good mechanical and dielectric properties， which are suitable for special-shaped and large scale component such as radome.