The effect of several important processing variables was investigated on formation of porous mullite with acicular microstructure. Experimental results demonstrated that microstructure and porosity of porous mullite are depending on concentration of $AlF_3$, holding time at $900^{circ}C$ and starting material. Acicular mullite was developed by increasing amount of $AlF_3$ and holding time at $900^{circ}C$. Mullite began to be formed at $1200^{circ}C$ and the resultant microstructure sintered at this temperature is similar to those at higher temperatures. Porosity increases with increase in amounts of $AlF_3$ and holding time at $900^{circ}C$ . Therefore, it is found that microstructure of reaction-sintered porous mullite can be controlled by governing the amount of $AlF_3$ and holding time at $900^{circ}C$.