Mullite powder was prepared through reaction of aluminum sulfate and silica in molten sodium sulfate media. The phase composition and microstructure of mullite powder were investigated. The results showed that high purity mullite is prepared by this method. The mullite powder exhibits needle-like whisker crystal with 50~100 nm in diameter and 3~8 #m in length. It is found that by this method mullite starts to form at 900oC, the quartz phase is disappeared about 1000oC, and mullite begin to decompose when the temperature is over 1100oC. Thus, the reasonable synthesizing temperature is about 1000oC. This synthetic method is characterized by process simplicity, the temperature of mullite formation is lower than 200-400oC compared with other conventional methods.