Microstructural effects on the mechanical and thermomechancical properties of mullite ceramics have been investigated. The mullite ceramics were fabricated by reaction-sintering of mixture of kaolin and Al₂O₃(M-1) and by pressureless-sintering of a powder prepered from Al₂O₃ and SiO₂ sols (M-2) and Al₂O₃ sol-ethylsilicate (M-3) system from the sol-gel method. Transmission electron microscopic observation revealed that samples M-1 and M-2 contained foreign phases, but that they were much less in M-2. These showed different temperature dependence in mechanical properties such as hardness, Young's modules, fracture toughness and strength. These differences were explained by the amount of grain boundary impurity phases. Thermal shock fracture behavior was also discussed.