The thermal analysis of $Al(OH)_3-SiO_2-AIF_3$ system was done. The thermal behaviors of components and the effect of moisture on the formation of mullite were investigated via TG-DTA and XRD analysis. The mixture of $Al(OH)_3,;SiO_2,;AIF_3$showed two endothermic peaks with drastic weight loss and one exothermic peak. Fluorotopaz was formed at $800^{circ}C$ and turned into mullite completely at $1100^{circ}C$. But the mixture $Al_2O_3$ of or dehydrated $Al(OH)_3$ and $SiO_2$ could not form mullite even at $1200^{circ}C$ in which the crystalline phases were $alpha-Al_2O_3$ ana cristobalite. It was found that the synthesized mullite was aciculate shaped single crystalline whisker.