Mullite/SiC composite ceramics containing 20 vol% SiC particles (0.12-1.71μm) were prepared by hot-pressing at 1650°C under 35 MPa for 4h. The effect of SiC particle size on mechanical properties such as bending strength and plastic deformation behavior at high temperatures was investigated as a function of cross-head speed. Bending strength and yield stress of mullite/SiC composite ceramics were higher than those of monolithic mullite at high temperatures. Bending strength and yield stress at 1300°C decreased with decreasing cross-head speed. At 1300°C, mullite/SiC composite ceramics with SiC grain size of 0.56-0.76 μm showed the highest bending strength and yield stress, which are 30-80% higher than those of monolithic mullite. On the other hand, mullite/SiC composite ceramics with SiC grain size of 0.18 μm showed maximum plastic deformation.