Silicon nitride (Si3N4) has been researched intensively because of superior mechanical properties up to high temperature. The mechanical properties of Si3N4 are strongly related to microstructure. The microstructure control of silicon nitride is well known to be a key issue for tailoring the mechanical properties of structural ceramics. This work was performed to reveal the effect of microstructure on dielectric properties at microwave frequency. Three starting powders were used fine, course a-Si3N4 and b-Si3N4. Sintering additives, 5 wt.% Y2O3, 2 wt.% Al2O3 and 1 wt.% MgO were mixed with each starting powder. Si3N4 ceramic with different b/a phase specimen were obtained by hot pressing. The post-resonator method was used for the measurement of dielectric properties, dielectric constant (e′) and dielectric loss (tand), at microwave frequency range. Silicon nitride ceramics show dielectric constant of 8.1 – 8.6 and dielectric loss 1.1 x 10-3 – 5.6 x 10-3. The effect of grain size and the role of phase on microwave dielectric properties are discussed.