The effect of addition of nano-sized particles on densification and thermal conductivity of AIN ceramics was investigated. The commercially available AIN powder (similar to 0.9 mu m) was mixed with 1.89 mass% nano-sized AIN particles (< 0.1 mu m), 3.53 mass% Y2O3, and 2.0 mass% CaO as sintering aid. The mixture was fired at 1500 degrees and 1600 degrees C in a tungsten resistance furnace under flowing N-2 atmosphere. The results showed that a fully densified specimen was obtained at the lower temperature of 1600 degrees C by addition of nano-sized particles. The thermal conductivity of the resulting product was 133 W/m degrees C. The value is much higher than the 52 W/m degrees C for the sample prepared without adding the nano-sized AIN powder. This study indicates a strong potential for the use of nano-sized particles as additives in the densification of AIN ceramics.