In order to fabricate aluminum nitride/boron nitride (AlN/BN) nanocomposites by pressureless sintering, the present study investigated the synthesis of AlN-BN nanocomposite powders by carbothermal reduction and nitridation of aluminum borate powders. Homogeneous mixtures of alumina (Al2O3), boric acid (H3BO3), and carbon powder were used to synthesize AlN/BN nanocomposite powders containing 10 and 20 vol% BN. Aluminum borate was produced by reacting Al2O3 and B2O3 above 800 °C, and AlN and turbostratic BN (t-BN) were produced by reacting aluminum borate with carbon powder and nitrogen gas at 1500 °C. Carbothermal reduction followed by nitridation yielded an AlN/BN nanocomposite powder composed of nanosized AlN and t-BN. By pressureless sintering nanocomposite AlN/BN powders containing 5 wt% Y2O3, AlN/BN nanocomposites were obtained without compromising the high thermal conductivity and high hardness.