Mullite whiskers were synthesized by the reaction of fly-ash and alumina powder. During the reaction process, the effect of oxide additives on the formation of mullite whisker was investigated. Mullite whisker was synthesized above 1350oC with the addition of excess alumina, and the aspect ratio of mullite whisker increased with increasing sintering temperature and time. Also, the aspect ratio of mullite whisker is dependent on the liquid phase based on silicate composition. The effect of various oxide additives on the aspect ratio of mullite whisker was investigated.