Mullitization in a multicomponent oxide system(alumina-kaolin-quartz-feldspar-talc) was studied as a function of sintering temperature from 1200 to 1500$^{circ}C$ based upon a quantitative X-ray diffraction analysis. In the present study mullite grew as wiskers and its formation reaction showed characteristic there stages as follows In the first stage(1255-1295$^{circ}C$) an appreciable mullitization(nucleation) occurred while corun-dum dissolution into glass (increasing glass content ) limited the rate of the reaction. At 1295-1335$^{circ}C$ (second state) the reaction was significantly enhanced with a considerable glass consumption and with no appreciable change in corundum content. Finally (above 1335$^{circ}C$) the reaction rate was attenuated re-markably with an apparent decrease in glass consumption rate. The impingement of mullite whiskers by oth-er whiskers and crystals was speculated to cause mullite growth in thickness direction with a slow growth rate resulting in the diminished reaction rate in the final stage.