Diffusion bonding of fine-grained mullite and ZrO2-toughened mullite was performed in the temperature range from 1500 to 1550 °C in air. Uniaxial pressure was applied at high temperature during the bonding process. The surface roughness to be bonded (Rmax) was about 3 μm. Bonding strength was measured by four-point bending tests and the strength of the base material was measured by three-point bending tests. The effects on the bonding strength of bonding conditions such as temperature and applied strain were examined. Bonding strength increased with increasing bonding temperature and applied strain. The bonding strength of mullite and ZrO2-toughened mullite was about 80% of the strength of the base material before bonding. The bonding strength of mullite was maintained up to 1000 °C.