This research presented the kinetics of mullite formation in fly ash-bauxite reactants couples. Experiment on isothermal conversion of fly ash-bauxite at 1100°C, 1300°C, 1500°C has been carried out. XRD was used to quantitatively measure the content of mullite specimens sintered at different temperatures and times. The kinetics curve of conversion ratio versus time has been drawn. The results obtained showed that the full transformation of fly ash-bauxite to mullite takes place between 1300 °C and 1500 °C. The activation energy of secondary mullite formation is deduced to be 151 kJ/mol in the range of 1100-1500°C. The growth morphology parameter, n, is about 1.14 and 0.45 at 1100°C and 1500°C, respectively, indicating that bulk nucleation is dominant in mullite crystallization followed by three-dimensional growth of mullite crystal controlled by diffusion, but at 1500 °C diffusion process dominates mullite formation process.