Cordierite composed of an alumina-silica-magnesia compound has a low coefficient of thermal expansion(CTE) and excellent thermal shock resistance. It also has a low dielectric constant and high electrical insulation. However, due to low mechanical strength, it is limited for use in a ceramic heater. In this study, ZrO2 is added to an 80 wt% cordierite-20 wt% mullite composition, and the effect of ZrO2 addition on the mechanical strength and thermal shock resistance is investigated. With an increasing addition of ZrO2, cordierite-mullite formed ZrO2, ZrSiO4 and spinel phases. With sintering conducted at 1400 °C with the addition of 5 wt% ZrO2 to 80 wt% cordierite-20 wt% mullite, the most dense microstructure forms along with an excellent mechanical strength with a 3-point flexural strength of 238MPa. When this composition is quenched in water at ΔT = 400 °C, the 3-point flexural strength is maintained. Moreover, when this composition is cooled from 800 °C to air, the 3-point flexural strength is maintained even after 100 cycles. In addition, the CTE is measured as 3.00 × 10-6·K-1 at 1000 °C. Therefore, 80 wt% cordierite-20 wt% mullite with 5 wt% ZrO2 is considered to be appropriate as material for a ceramic heater.