To develop new composites with sufficiently high thermal conductivity and suitably controlled D-k value for PCBs application, the composites were prepared from epoxy and AlN or BN fillers, and the effects of content, size, size distribution, and morphology of two fillers on the thermal and dielectric properties of the composites were investigated. The results showed that either AlN or BN fillers can greatly increase T-g and thermal conductivity, decreasing CTE and D-f, and suitably controlling the increase of D-k. At the same filler content, BN-filled composites exhibit better thermal performance and dielectric properties compared to AlN-filled composites. In the case of BN-filled composites, it is found that platelet-shaped micro-BN filled composite has lower T-g and higher CTE compared with particle-shaped nano-BN filled composite, but its thermal conductivity is remarkably higher than that of nano-BN filled composite. When hybrid BN fillers are used, thermal conductivity further increases. For micro- or nano-BN filled composite, the composite shows decreased T-g and increased D-k at high fraction of BN, but hybrid BN-filled composite still has high T-g and similar D-k with epoxy even if at high fraction of BN. Compared with single-sized AlN-filled composite, it is found that hybrid-sized AlN-filled composite has higher T-g and lower CTE, but has lower thermal conductivity. To predict thermal conductivity and D-k in the investigated materials system, different models reported in the literature were analyzed and compared with the experimental data. Finally, suitable models were recommended. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2754-2764, 2010