Polymer-matrix composites based on brominated epoxy as the matrix and aluminum nitride (AlN) particle as the filler were prepared. Effects of AlN size and content as well as composite processing conditions on the preparation and properties of the composites had been investigated. At the same processing conditions, Young's modulus (E) and dielectric constant (Dk) of the composites increase, whereas coefficient of thermal expansion decreases when increasing AlN content or decreasing AlN size; tensile strength and elongation at break first increase then decrease with AlN content, and they reach maximum values at lower AlN content with decreasing AlN size; glass transition temperature (Tg) also exhibits a trend of first increase then decrease with AlN content, and it decreases with decreasing AlN size, especially at high AlN content; dissipation factor (Df) generally decreases with AlN content except for the composites filled with 50 nm-AlN, and it increases with decreasing AlN size. Comparing the composites prepared at different processing conditions, the properties of the composite are relatively poor at low vacuum conditions during removal of solvent and bubble. The scanning electron microscope and Fourier transform infrared analyses indicate that the properties of the composites are related to the aggregation of AlN filler and voids in the composites as well as the crosslink density of epoxy matrix. The preparation of the composites is also found to be affected by AlN size and content as well as vacuum conditions, indicating that increase of viscosity of system and/or the solvent evaporation during curing results in poor formability of the composites. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013