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Abstract: A kind of polymer composite was fabricated using polystyrene as the matrix and Si3N4 powder as filler employing the method of heat press molding. Microstructure, thermal conductivity and dielectric constant of the Si3N4 filled composite were evaluated. The effect of the volume fraction of Si3N4, the particle size of the polystyrene matrix and the silane treatment of Si3N4 filler on the thermal conductivity of the composite was investigated; dielectric constant of the composite was evaluated. The main factors that affect the thermal conductivity of the composite were confirmed through theoretical analyzing of the experimental data and the thermal conductivity model. Experimental results show that with the filler content increasing, a thermally conductive network is formed in the composites, thus the thermal conductivity of the composite increases rapidly. The composites experience a highest thermal conductivity of 3.0W/mK when the volume fraction of the filler reaches 40%. The increasing of thermal conductivity is dominated by the ease of forming a thermal conductive network. A larger polystyrene particle size, a higher Si3N4 filler content and the silane treatment of the filler have a beneficial effect on improving the thermal conductivity. The dielectric constant increases with the content of Si3N4 filler, however, it remains at a relatively low lever (<4, at 1MHz). [Copyright &y& Elsevier]