Abstract: The influence of Si₃N₄ content and relative density on dielectric properties of SiO₂-Si₃N₄ composites was studied. It is found that in the same Si₃N₄ content, dielectric constant of the composites is increased and loss tangent is reduced when the relative density of the composites is increased;in a range of relative density, dielectric constant and loss tangent of the composites are increased when the Si₃N₄ content is increased.