A novel approach to prepare dense AlN ceramics with high thermal conductivity was proposed. The aluminum isopropoxide (AIP) was introduced to prepare dense AlN ceramics possessing enhanced thermal conductivity via in situ reaction. The effect of AIP and sintering temperatures on the microstructure and properties of AlN ceramics were investigated. Results indicate that AIP is beneficial for the densification of AlN ceramics, and particularly, when the addition of AIP reaches 1 wt%, the density and thermal conductivity could reach up to 3.29 g/cm3, 182 W/mK, respectively. Finally, the mechanism of the in situ reaction was proposed.