Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y 2 O 3 ) as sintering aid. Effects of Y 2 O 3 content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y 2 O 3 in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y 2 O 3 content from 3wt.% to 8wt.% into AlN/15vol.% BN ceramics, the thermal conductivity increased from 110W/mK to 141W/mK.