Mullite whiskers were produced by firing of kaolin at 1400degrees-1550degreesC. CuSO4 was used as fluxing agent. Before firing, spray drying and baking with flour and leaven were employed to give free space to the powders. The influence of preparation parameters and firing temperature on the crystalline structure and phase transformations during the entire process was determined. The results showed that there was a beneficial effect of both the incorporation of CuSO4 and the given free space on the formation of mullite whiskers. There was no evidence for the effect of firing temperature on the aspect ratio and morphology of the whiskers within the investigated range.