Currently, as mordants, heavy metals are being used in dyeing process in order to improve dye uptake and dye fastness. These chemicals, however, are mostly harmful to human body and the environment along with the pollution by effluent. Traditionally, rice straw ash solution, lime, etc. have been used as mordants in order to improve the effects on dye uptake and color fastness of dyestuffs. The study of natural mordants would, therefore, be vital to the development and succession of our traditional dyeing methods. In this study, as a preliminary step toward the use of rice straw ash solution for the nautral dyeing using Caesalpinia sappan, several aluminum compounds were employed as mordants. When the purified aluminum compounds were used, on the basis of unmordanted cotton fabric, the K/S values of the pre-mordanted and simultaneously mordanted fabrics increased by 2 or 3 times, while those of the post-mordanted fabrics decreased.