The effect of addition of nano‐sized particles on densification and thermal conductivity of AlN ceramics was investigated. The commercially available AlN powder (∼0.9 μm) was mixed with 1.89 mass% nano‐sized AlN particles (<0.1 μm), 3.53 mass% Y2O3, and 2.0 mass% CaO as sintering aid. The mixture was fired at 1500° and 1600°C in a tungsten resistance furnace under flowing N2 atmosphere. The results showed that a fully densified specimen was obtained at the lower temperature of 1600°C by addition of nano‐sized particles. The thermal conductivity of the resulting product was 133 W/m°C. The value is much higher than the 52 W/m°C for the sample prepared without adding the nano‐sized AlN powder. This study indicates a strong potential for the use of nano‐sized particles as additives in the densification of AlN ceramics.