ZTA composite ceramics were prepared via pressureless sintering, and the influence of Cr2O3 doping on the microstructure and mechanical properties of ZTA ceramics was investigated. The results show that Cr2O3 can promote the growth of Al2O3 grains and obtain elongated Al2O3 grains. The density, hardness and fracture toughness of ZTA ceramics firstly increase and then decrease with the increase of Cr2O3 addition. ZTA ceramics with 0.6% Cr2O3 addition exhibit the optimum properties. However, the properties of ZTA ceramics with the excessive addition of Cr2O3 can be impaired due to the increased internal porosities. The mechanical properties of ZTA composite ceramics with Cr2O3 nano-sized particles are superior to those with Cr2O3 micron-sized particles. The addition of Cr2O3 nano-sized particles results in greater density and hardness of ZTA ceramics due to the smaller average grain size of Al2O3. Besides, the fracture toughness of ZTA ceramics improves due to the formation of more elongated grains and the consumption of more energy for crack propagation.