AlN with high thermal conductivity was fabricated by pressureless sintering with Y2O3 as the sintering aid. The thermal conductivity was observed to increase with sintering time (up to 8 h) at 1810 degrees C. The distribution of the sintering aid was identified as one of the major factors influencing the thermal conductivity in AlN. Non-uniform distribution of the grain boundary phase was found to be associated with a significant amount of porosity, resulting in the enhancement of phonon scattering and thereby lowering the thermal conductivity.