Si-Al-Zr-O (SAZ) composite ceramics with ultra fine Grains were prepared by in-situ controlled crystallizing from SAZ amorphous bulk. The crystallization behaviors of SAZ amorphous bulk were investigated by differential scanning calorimetric (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The results show that the SAZ amorphous bulk starts devitrifying at 900~950℃ and t-ZrO₂ phase forms first. With the increase of temperature up to 1000℃, mullitization occurs and mullite becomes major phase at 1100℃. A small amount of t-ZrO₂ transform to m-ZrO₂ for the samples treated at 1100℃ and more than half of t-ZrO₂ transform to m-ZrO₂ for the samples treated at 1250℃. The sample heated at 1100℃ for 1h shows very dense and homogenous microstructure with the size of ball-like grains about 20~50 nm. With the increase of temperature up to 1350℃, the grains grow quickly and some grow into lath-shaped grains with major diameter about 5 μm. The highest flexural strength and fracture toughness of the samples treated at 1150℃ for 1 h followed by pretreating at 950℃ for 2 h, are 520 MPa, 5.13 MPa·m^1/2 , respectively.