Thermally conducting, but electrically insulating, polymer-matrix composites exhibiting low dielectric constant are needed for electronic packaging. For developing such composites, this work used silicon nitride particles as fillers and epoxy as matrix. The thermal conductivity of Si3N4 particles epoxy-matrix composites was increased by up to 31.4 times than that of neat polymer by silane surface treatment of the particles prior to composites fabrication. The increase in thermal conductivity is due to decrease in the filler-matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 45.4 vol. % silane-treated Si3N4 particles only, the thermal conductivity of epoxy-matrix composites reached 9.72W/ (m*K). The dielectric constant was also low (up to 5.0 at 1 MHz). However, Si3N4 addition caused the flexural strength and ductility to decrease from the values of the neat polymer.