In this work, a new route for possibility synthesizing high purity submicron size mullite (3A12O3.2SiO2) powder at relatively low temperature. Mullite precursor has been prepared from mixing precipitated silica powder with solution of aluminum nitrate nonahydrate (Al(NO3)3.9H2O) through sol-gel process, in order to obtain composite powder. The formed precursor gel was calcined at temperatures of (1100, 1150, 1200 and 1250)°C for 1 hour. The influence of calcination temperatures of the mullite synthesis was discussed. The microstructure tests of calcined materials were investigated by x-ray diffractometer (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX). X-ray analysis results identified the mullitization initiated at 1200°C, and the mullite phase was completely formed at 1250°C. The SEM micrographs show the microstructure of mullite powder had been aggregated in the regular and nearly spherical-like appearances with diameters in the range of (100-200) nm.