The influence of phase formation on the dielectric properties of silicon nitride (Si3N4) ceramics, which were produced by pressureless sintering with additives in MgO-Al2O3-SiO2 system, was investigated. It seems that the difference in the dielectric properties of Si3N4 ceramics sintered at different temperatures was mainly due to the difference of the relative content of alpha-Si3N4, beta-Si3N4, and the intermediate product (Si2N2O) in the samples. Compared with alpha-Si3N4 and Si2N2O, beta-Si3N4 is believed to be a major factor influencing the dielectric constant. The high-dielectric constant of beta-Si3N4 could be attributed to the ionic relaxation polarization.