Epoxy/AlN composites were prepared using high purity AlN particles with an average size of 1.1 μm. AlN particles were surface-treated with three kinds of silane coupling agents and three kinds of surface-modifiers. Dielectric properties and thermal conductivity were evaluated. As a result, it was found that silane coupling agents with epoxy, mercapto, and amino groups create higher thermal conductivity than surface modifiers such as GO, POSS and hyper-branched polymer. A silane coupling agent with mercapto group and a graphene oxide surface modifier are promising at high and low loading levels, respectively. From the dielectric spectroscopy study, it can be said that interfacial conditions should be improved by finding out proper methods.