Mullite whiskers were synthesized by a vapor-solid reaction. Mullite composition xerogels were fired at 900" to 1600°C with AIFj in an airtight container. An average whisker increased in length from 7 pm at 1100°C to 10 pm at 16OO0C, whereas an average whisker decreased in aspect ratio from 25 to 10 with increased firing temperature. The whiskers elongated to the c-axis and the side planes were the (110). A clear lattice image corresponding to (110) lattice spacing up to the edges of the whiskers was observed with high-resolution electron microscopy, and no droplet was observed on the tips of the whiskers. The chemical composition of the whiskers synthesized below 1100°C showed an apparent A120,-rich composition of about 72 mol%. Composites reinforced by 15 vol% of mullite whiskers in the matrix of 75 vol% mullite/25 vol% Y-TZP enhanced the fracture toughness compared with those materials without mullite whiskers. [