High-energy ball milling was applied to a commercial mullite powder, which had a particle size ranging from 2 to 10 mu m, and was considerably refined to be < 1 mu m after milling for 10 h. Anisotropic grain growth of mullite was observed in the milled mullite powder, starting at relatively low temperatures (1200 degrees C). The anisotropic grain growth was possibly attributed to the special anisotropic structure of mullite and refined mullite powder as a result of the high-energy ball milling, although other factors, such as lattice defects, localized stress of the milled powder, and impurities or contaminations caused by the high-energy milling also could be factors. This anisotropic grain growth of mullite might be useful in the fabrication of reinforced ceramics based on mullite and other materials.