The fabrication processing, dielectric properties and thermal properties of polymer-matrix composites containing AlN particles (10μm) for electronic substrates and microelectronic packaging applications were investigated. The epoxy resin (E-51) is used as the matrix, and the dispersion of the AlN in the composites is varied form 0 vol% to 40 vol%. The microstructures of the polymer-matrix composites are observed through scanning electron microscopy (SEM). With increasing the AlN content, thermal conducting of composites is improved, while the composites still keep the relatively low dielectric constant and dielectric loss. According to the dielectric properties dependence on frequencies (1kHz-10MHz) of the composites, the Cole-Cole plot is analyzed. The dipole relaxation in the composite is induced, and it is suggested that the air layer exit between the epoxy resin and the AlN particles.