High thermal conductivity aluminum nitride (AIN) substrates were prepared by aqueous tape casting. The characteristics of surface-treated AIN powder were studied in aqueous ball-milling media. The oxygen content of AIN powder with the dispersant was lower than that of AIN powder without the dispersant at the same ball-milling time. The isoelectric points of the surface-treated AIN with and without a dispersant were, respectively, at pH similar to 3.35 and pH similar to 3.90. The atmosphere had considerable effects on organic additive burnout of aqueous AIN green sheets. The composition of grain boundaries changed with increasing holding time at 1850 degrees C. A translucent AIN substrate with a uniform microstructure and a thermal conductivity of 263 W (.) (m (.) K)(-1) was obtained by pressureless sintering at 1850 degrees C for 6 h in a nitrogen atmosphere.