Abstract: AlN ceramics were fabricated by hot-pressing with Y2O3 as sintering additive added by direct addition and in-situ formation. The effects of adding method and amount of sintering additive on microstructure and mechanical properties of AlN ceramics were investigated. The results show that the sintering additive added by in-situ formation was more beneficial to densification of AlN ceramics, and particularly, when Y2O3 was added by in-situ formation and reached 2wt%, the relative density, hardness and flexural strength could reach up to 990%, 15.39 GPa and 383.0 MPa, respectively, higher than those when Y2O3 was added directly. At the same time an integrated-shaped microstructure with low content of secondary phase, which mainly distributed at trigeminal grain boundaries, and with surface contact between grains was generated. However, with the increase of adding amount of the sintering additive added by in-situ formation, the relative density, hardness and flexural strength of AlN ceramics decreased due to the large amount of secondary phase at grain boundaries.