AlN ceramics were prepared by hot pressing sintering process with AlN powder prepared by direct nitriding method as raw material and Y2O3 with mass fraction of 5% as sintering assistant. The effects of sintering temperature and pressure on the microstructure, relative density and thermal conductivity of AlN ceramics were studied. The results showed that with the increase of temperature, the grain size of AlN ceramics grew, and the second phase gradually increased. The thermal conductivity and relative density of AlN ceramics first increased and then decreased. With the increase of pressure, the grain size of AlN ceramics gradually became smaller, the porosity decreased, and the thermal conductivity and relative density increased significantly. The optimal sintering conditions of AlN ceramics were as follows: the temperature of 1800 ℃, and the pressure of 50 MPa.