Mechanical properties of hot-pressed mullite-SiC whisker composites were studied in relation to the whisker content and chemical composition of matrix. Grain growth of mullite was suppressed by the existence of SiC whisker in the matrix, but the densification was not inhibited by the loading of whisker up to 30 vol%. The room temperature flexural strength of specimens was improved by the addition of SiC whisker in every chemical compositions of the matrix examined, more pronouced with increasing SiO2 content in the matrix. The high temperature strength was almost the same as that at room temperature up to 1300°C. In specimens containing a glassy grain-boundary phase, addition of SiC whisker to mullite did not significantly affect KIC value. Combination of single phase mullite (74 wt% Al2O3) and SiC whisker gave the best effect on the KIC value both at room temperature and elevated temperature. These phenomena can be explained by the presence of a glassy grain-boundary phase, which causes the matrix to the whisker bonding.