Aluminum nitride (AlN) ceramics, with binary additives B2O3-Y2O3, were sintered at temperatures from 1700 to 1850 degrees C. The microstructure and sintering characteristics were studied by XRD, HREM, SEM and TEM/EDS, which showed that Y2O3 gave different yttrium aluminates through the reaction with Al2O3 under different conditions. With the increase of sintering temperature, the yttrium-to-aluminum atomic ratio Y/Al decreased in the secondary phases of the sintered bodies. It was discovered that B2O3 could dissolve in the yttrium aluminates, forming some ordered structure with a superlattice. After sintering at 1850 degrees C for 4 h, a specimen with a fine microstructure and a thermal conductivity of 190 W m(-1) K-1 was obtained.