In the current study, the effects of loading level of micro or nano size BN particles on the thermal, mechanical, and morphological properties of silicone rubber are investigated. Three micron size and two nano size BNs with different particle sizes and shapes are used. All five types of BNs are found to be well-dispersed in silicone rubber matrix despite some local agglomerates. In general, the addition of BN particles in silicone matrix decreases the tensile strength and strain at break, coefficient of thermal expansion (CTE) values, on the other hand increases modulus, hardness and thermal conductivity. Nano size fillers have more pronounced effect on tensile properties of composites in comparison to micron size BNs at any given loading level. The aspect ratio of the filler is found to be very effective in achieving high thermal conductivity in composite systems. Dielectric constants of composites vary between dielectric constant of silicone and BN. (C) 2009 Elsevier B.V. All rights reserved.