Ultrafine mullite powder was prepared from aluminium-secbutoxide and tetraethyl orthosilicate(TEOS) in the molar $Al_2O_3/SiO_2$=3/2. Sol-gel method by partial hydrolysis technique, as it were, first, TEOS was partially hydrolysized and then mixed with Al-secbutoxide for complete hydrolysis was used. X-ray diffraction, infrared spectroscopy and transmission electron microscopy, etc. confirmed that the mullite powder prepared by this method is in the stoichiometric $Al_2O_3/SiO_2$ ratio. Al-Si spinel was formed at $980^{circ}C$ and ultrafine mullite powder with about 20 nm particle size was obtained above $1,200^{circ}C$. Also mullite powders calcined at $1,600^{circ}C$ had a stoichiometric composition, $3Al_2O_3{cdot}2SiO_2$ and the lattice constants of the mullite powders calcined above $1,200^{circ}C$ were almost coincided with theoretical values.