ZrO2-mullite nano-composite ceramics were prepared by in-situ crystallization of Si-Al-Zr-O (SAZ) amorphous bulk. The amorphous bulk and as-prepared ceramics were characterized by XRD, IR and SEM. The influence of heat-treatment temperature on the structure and mechanical properties of ZrO2-mullite nano-composite ceramics was investigated. The results show that the t-ZrO2 began to form after 960 oC heat treatment of SAZ bulk and mullite was crystallized at 1000 oC in the amorphous bulks. The main phases were identified as t-ZrO2 and mullite, and at the same time cristobalite formed by a two-step gradient heat treatment at 950-1000 oC and 1100-1250 oC. The first-step nucleation temperature has a great effect on the microstructure and fracture toughness, and with the increase of the temperature from 950 oC to 1000 oC, the nano-particle size increases but the fracture toughness decreases sharply. However, the influence of the second-step crystallization temperature on the mechanical properties is not obvious. The fracture toughness and flexural strength increase gradually, and then decreases as the crystallization temperature increasing. The nano-composite ceramics exhibit the best mechanical properties, including the flexural strength up to 521 MPa and the fracture toughness up to 5.13 MPa·m1/2 after 950 oC nucleation and then 1150 oC crystallization