Mullite ceramics are stable at high temperature, and expected for structural applications at high temperature. But the mullite ceramics have low fracture toughness. In this study, method to optimize the microstructure for high toughness in Si3N4 system was used as a reference, and microstructure of mullite ceramics were controlled to have needle-shaped grains with narrow size distribution. As a result, fracture toughness of the mullite ceramics was improved. At first, we synthesized an amorphous mullite powder containing Y2O3 by the coprecipitation method. The powder mixture of amorphous mullite and crystallized mullite was sintered at 1600°-1700°C. It was found that mullite whiskers and crystallized Y2Si2O7 precipitated in the sintered body containing Y2O3. Its fracture toughness increased and the bending strength showed no degradation at high temperature.