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The creep deformation in 72 wt%/Al_2O_3-mullite (72A), 74 wt%Al_2O_3-mullite (74A) and those containing 10 vol%ZrO_2 (72A-10Z and 74A-10Z, respectively) has been studied. Stress exponent n's for creep in 72A-10Z and 74A-10Z were 1.1-1.2 and 1.0-1.2, respectively, slightly smaller than those in 72A (1.2-1.3) and 74A (1.3-1.5). Activation energies for creep in 72A-10Z and 74A-lOZ were 157 and 158 kcal/mol, respectively, slightly smaller than those in 72A and 74A (171 and 169 kcal/mol, respectively). Creep rate in 74A was increased more than that in 72A by addition of 10 vol%ZrO_2. It was estimated that the enhanced creep rate of mullite ceramics by the addition of ZrO_2 was ascribed to the relaxation of the stress at grain boundary triple points by the creep deformation of ZrO_2 grains.