The exsolution assemblage of sillimanite from quartzofeldspathic gneiss in the Larsemann Hills of East Antarctica consists of magnetite, ilmenite-hematite and quartz exsolution laminae and minor orthopyroxene. A compositional analysis shows that the sillimanite primarily crystallized at high temperature has not only Fe3+ but also Fe2+, Mg2+ and Ti4+ substitutes. Iron oxides can be as high as 2.9%. With the decreasing temperature, the solubility of the other components in sillimanite decreased and most of the dissolved components were exsolved, resulting in the formation of the exsolution texture. The sill imanite stable at low temperature is dominated by Fe3+ substitution for Al in the octhedral coordination. The iron replacement of sillimanite may somewhat affect the physic-chemical calculation and exert negative effects on sillimanite mineralization. The Fe substitution in sillimanite has an insignificant effect on a but a substantial effect on b, especially on c values of the crystal cell lattice. These results are to some extent contradictory to the previous conclusions.