Mullite formation in a porcelain body was promoted extensively by replacing kaolinite with pyrophyllite. Effects of mullite formation and vitrification by substitution of kaolinite with pyrophyllite on the mechanical and thermal properties were investigated. Addition of 45-55% pyrophyllite (pyrophyllite (45-55%)-feldspar (30%)-Gairome clay (20%)) could vitrify the sintered samples (water absorption : 0.05%, bulk density : 2.66g/cc) and improve the flexural strength (122MPa) when fired at $1280^{circ}C$. Mullite formation was found to be decreased with increasing content of pyrophyllite. On the contrary, beyond 50% of pyrophyllite quartz and cristobalite phases was found to be increased. Thermal expansion coefficient of the samples decreased with increase of mullite phase. In triaxial system of pyrophyllite-feldspar-clay, the mullite formation of the samples with 50% pyrophyllite reaches about 78.7% and thermal expansion coefficient was found to be $5.4{ imes}10^{-6}/K$.